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Secure Programming for Linux and Unix HOWTO

Chapter 6. Avoid Buffer Overflow


An enemy will overrun the land; he will pull down your strongholds and plunder your fortresses.

 Amos 3:11 (NIV)

An extremely common security flaw is vulnerability to a ``buffer overflow''. Buffer overflows are also called ``buffer overruns'', and there are many kinds of buffer overflow attacks (including ``stack smashing'' and ``heap smashing'' attacks). Technically, a buffer overflow is a problem with the program's internal implementation, but it's such a common and serious problem that I've placed this information in its own chapter. To give you an idea of how important this subject is, at the CERT, 9 of 13 advisories in 1998 and at least half of the 1999 advisories involved buffer overflows. An informal 1999 survey on Bugtraq found that approximately 2/3 of the respondents felt that buffer overflows were the leading cause of system security vulnerability (the remaining respondents identified ``mis-configuration'' as the leading cause) [Cowan 1999]. This is an old, well-known problem, yet it continues to resurface [McGraw 2000].

A buffer overflow occurs when you write a set of values (usually a string of characters) into a fixed length buffer and write at least one value outside that buffer's boundaries (usually past its end). A buffer overflow can occur when reading input from the user into a buffer, but it can also occur during other kinds of processing in a program.

If a secure program permits a buffer overflow, the overflow can often be exploited by an adversary. If the buffer is a local C variable, the overflow can be used to force the function to run code of an attackers' choosing. This specific variation is often called a ``stack smashing'' attack. A buffer in the heap isn't much better; attackers may be able to use such overflows to control other variables in the program. More details can be found from Aleph1 [1996], Mudge [1995], LSD [2001], or the Nathan P. Smith's "Stack Smashing Security Vulnerabilities" website at A discussion of the problem and some ways to counter them is given by Crispin Cowan et al, 2000, at A discussion of the problem and some ways to counter them in Linux is given by Pierre-Alain Fayolle and Vincent Glaume at

Most high-level programming languages are essentially immune to this problem, either because they automatically resize arrays (e.g., Perl), or because they normally detect and prevent buffer overflows (e.g., Ada95). However, the C language provides no protection against such problems, and C++ can be easily used in ways to cause this problem too. Assembly language also provides no protection, and some languages that normally include such protection (e.g., Ada and Pascal) can have this protection disabled (for performance reasons). Even if most of your program is written in another language, many library routines are written in C or C++, as well as ``glue'' code to call them, so other languages often don't provide as complete a protection from buffer overflows as you'd like.

6.1. Dangers in C/C++

C users must avoid using dangerous functions that do not check bounds unless they've ensured that the bounds will never get exceeded. Functions to avoid in most cases (or ensure protection) include the functions strcpy(3), strcat(3), sprintf(3) (with cousin vsprintf(3)), and gets(3). These should be replaced with functions such as strncpy(3), strncat(3), snprintf(3), and fgets(3) respectively, but see the discussion below. The function strlen(3) should be avoided unless you can ensure that there will be a terminating NIL character to find. The scanf() family (scanf(3), fscanf(3), sscanf(3), vscanf(3), vsscanf(3), and vfscanf(3)) is often dangerous to use; do not use it to send data to a string without controlling the maximum length (the format %s is a particularly common problem). Other dangerous functions that may permit buffer overruns (depending on their use) include realpath(3), getopt(3), getpass(3), streadd(3), strecpy(3), and strtrns(3). You must be careful with getwd(3); the buffer sent to getwd(3) must be at least PATH_MAX bytes long. The select(2) helper macros FD_SET(), FD_CLR(), and FD_ISSET() do not check that the index fd is within bounds; make sure that fd >= 0 and fd <= FD_SETSIZE (this particular one has been exploited in pppd).

Unfortunately, snprintf()'s variants have additional problems. Officially, snprintf() is not a standard C function in the ISO 1990 (ANSI 1989) standard, though sprintf() is, so not all systems include snprintf(). Even worse, some systems' snprintf() do not actually protect against buffer overflows; they just call sprintf directly. Old versions of Linux's libc4 depended on a ``libbsd'' that did this horrible thing, and I'm told that some old HP systems did the same. Linux's current version of snprintf is known to work correctly, that is, it does actually respect the boundary requested. The return value of snprintf() varies as well; the Single Unix Specification (SUS) version 2 and the C99 standard differ on what is returned by snprintf(). Finally, it appears that at least some versions of snprintf don't guarantee that its string will end in NIL; if the string is too long, it won't include NIL at all. Note that the glib library (the basis of GTK, and not the same as the GNU C library glibc) has a g_snprintf(), which has a consistent return semantic, always NIL-terminates, and most importantly always respects the buffer length.

Of course, the problem is more than just calling string functions poorly. Here are a few additional examples of types of buffer overflow problems, graciously suggested by Timo Sirainen, involving manipulation of numbers to cause buffer overflows.

First, there's the problem of signedness. If you read data that affects the buffer size, such as the "number of characters to be read," be sure to check if the number is less than zero or one. Otherwise, the negative number may be cast to an unsigned number, and the resulting large positive number may then permit a buffer overflow problem. Note that sometimes an attacker can provide a large positive number and have the same thing happen; in some cases, the large value will be interpreted as a negative number (slipping by the check for large numbers if there's no check for a less-than-one value), and then be interpreted later into a large positive value.
 /* 1) signedness - DO NOT DO THIS. */
  char *buf;
  int i, len;
  read(fd, &len, sizeof(len));
  /* OOPS!  We forgot to check for < 0 */
  if (len > 8000) { error("too large length"); return; }
  buf = malloc(len);
  read(fd, buf, len); /* len casted to unsigned and overflows */

Here's a second example identified by Timo Sirainen, involving integer size truncation. Sometimes the different sizes of integers can be exploited to cause a buffer overflow. Basically, make sure that you don't truncate any integer results used to compute buffer sizes. Here's Timo's example for 64-bit architectures:
 /* An example of an ERROR for some 64-bit architectures,
     if "unsigned int" is 32 bits and "size_t" is 64 bits: */
  void *mymalloc(unsigned int size) { return malloc(size); }
  char *buf;
  size_t len;
  read(fd, &len, sizeof(len));
  /* we forgot to check the maximum length */
  /* 64-bit size_t gets truncated to 32-bit unsigned int */
  buf = mymalloc(len);
  read(fd, buf, len);

Here's a third example from Timo Sirainen, involving integer overflow. This is particularly nasty when combined with malloc(); an attacker may be able to create a situation where the computed buffer size is less than the data to be placed in it. Here is Timo's sample:
 /* 3) integer overflow */
  char *buf;
  size_t len;
  read(fd, &len, sizeof(len));
  /* we forgot to check the maximum length */
  buf = malloc(len+1); /* +1 can overflow to malloc(0) */
  read(fd, buf, len);
  buf[len] = '\0';

6.2. Library Solutions in C/C++

One partial solution in C/C++ is to use library functions that do not have buffer overflow problems. The first subsection describes the ``standard C library'' solution, which can work but has its disadvantages. The next subsection describes the general security issues of both fixed length and dynamically reallocated approaches to buffers. The following subsections describe various alternative libraries, such as strlcpy and libmib. Note that these don't solve all problems; you still have to code extremely carefully in C/C++ to avoid all buffer overflow situations.

6.2.1. Standard C Library Solution

The ``standard'' solution to prevent buffer overflow in C (which is also used in some C++ programs) is to use the standard C library calls that defend against these problems. This approach depends heavily on the standard library functions strncpy(3) and strncat(3). If you choose this approach, beware: these calls have somewhat surprising semantics and are hard to use correctly. The function strncpy(3) does not NIL-terminate the destination string if the source string length is at least equal to the destination's, so be sure to set the last character of the destination string to NIL after calling strncpy(3). If you're going to reuse the same buffer many times, an efficient approach is to tell strncpy() that the buffer is one character shorter than it actually is and set the last character to NIL once before use. Both strncpy(3) and strncat(3) require that you pass the amount of space left available, a computation that is easy to get wrong (and getting it wrong could permit a buffer overflow attack). Neither provide a simple mechanism to determine if an overflow has occurred. Finally, strncpy(3) has a significant performance penalty compared to the strcpy(3) it supposedly replaces, because strncpy(3) NIL-fills the remainder of the destination. I've gotten emails expressing surprise over this last point, but this is clearly stated in Kernighan and Ritchie second edition [Kernighan 1988, page 249], and this behavior is clearly documented in the man pages for Linux, FreeBSD, and Solaris. This means that just changing from strcpy to strncpy can cause a severe reduction in performance, for no good reason in most cases.

Warning!! The function strncpy(s1, s2, n) can also be used as a way of copying only part of s2, where n is less than strlen(s2). When used this way, strncpy() basically provides no protection against buffer overflow by itself - you have to take separate actions to ensure that n is smaller than the buffer of s1. Also, when used this way, strncpy() does not usually add a trailing NIL after copying n characters. This makes it harder to determine if a program using strncpy() is secure.

You can also use sprintf() while preventing buffer overflows, but you need to be careful when doing so; it's so easy to misapply that it's hard to recommend. The sprintf control string can contain various conversion specifiers (e.g., "%s"), and the control specifiers can have optional field width (e.g., "%10s") and precision (e.g., "%.10s") specifications. These look quite similar (the only difference is a period) but they are very different. The field width only specifies a minimum length and is completely worthless for preventing buffer overflows. In contrast, the precision specification specifies the maximum length that that particular string may have in its output when used as a string conversion specifier - and thus it can be used to protect against buffer overflows. Note that the precision specification only specifies the total maximum length when dealing with a string; it has a different meaning for other conversion operations. If the size is given as a precision of "*", then you can pass the maximum size as a parameter (e.g., the result of a sizeof() operation). This is most easily shown by an example - here's the wrong and right way to use sprintf() to protect against buffer overflows:
 char buf[BUFFER_SIZE];
  sprintf(buf, "%*s",  sizeof(buf)-1, "long-string");  /* WRONG */
  sprintf(buf, "%.*s", sizeof(buf)-1, "long-string");  /* RIGHT */
In theory, sprintf() should be very helpful because you can use it to specify complex formats. Sadly, it's easy to get things wrong with sprintf(). If the format is complex, you need to make sure that the destination is large enough for the largest possible size of the entire format, but the precision field only controls the size of one parameter. The "largest possible" value is often hard to determine when a complicated output is being created. If a program doesn't allocate quite enough space for the longest possible combination, a buffer overflow vulnerability may open up. Also, sprintf() appends a NUL to the destination after the entire operation is complete - this extra character is easy to forget and creates an opportunity for off-by-one errors. So, while this works, it can be painful to use in some circumstances.

Also, a quick note about the code above - note that the sizeof() operation used the size of an array. If the code were changed so that ``buf'' was a pointer to some allocated memory, then all ``sizeof()'' operations would have to be changed (or sizeof would just measure the size of a pointer, which isn't enough space for most values).

The scanf() family is sadly a little murky as well. An obvious question is whether or not the maximum width value can be used in %s to prevent these attacks. There are multiple official specifications for scanf(); some clearly state that the width parameter is the absolutely largest number of characters, while others aren't as clear. The biggest problem is implementations; modern implementations that I know of do support maximum widths, but I cannot say with certainty that all libraries properly implement maximum widths. The safest approach is to do things yourself in such cases. However, few will fault you if you simply use scanf and include the widths in the format strings (but don't forget to count \0, or you'll get the wrong length). If you do use scanf, it's best to include a test in your installation scripts to ensure that the library properly limits length.

6.2.2. Static and Dynamically Allocated Buffers

Functions such as strncpy are useful for dealing with statically allocated buffers. This is a programming approach where a buffer is allocated for the ``longest useful size'' and then it stays a fixed size from then on. The alternative is to dynamically reallocate buffer sizes as you need them. It turns out that both approaches have security implications.

There is a general security problem when using fixed-length buffers: the fact that the buffer is a fixed length may be exploitable. This is a problem with strncpy(3) and strncat(3), snprintf(3), strlcpy(3), strlcat(3), and other such functions. The basic idea is that the attacker sets up a really long string so that, when the string is truncated, the final result will be what the attacker wanted (instead of what the developer intended). Perhaps the string is catenated from several smaller pieces; the attacker might make the first piece as long as the entire buffer, so all later attempts to concatenate strings do nothing. Here are some specific examples:

  • Imagine code that calls gethostbyname(3) and, if successful, immediately copies hostent->h_name to a fixed-size buffer using strncpy or snprintf. Using strncpy or snprintf protects against an overflow of an excessively long fully-qualified domain name (FQDN), so you might think you're done. However, this could result in chopping off the end of the FQDN. This may be very undesirable, depending on what happens next.

  • Imagine code that uses strncpy, strncat, snprintf, etc., to copy the full path of a filesystem object to some buffer. Further imagine that the original value was provided by an untrusted user, and that the copying is part of a process to pass a resulting computation to a function. Sounds safe, right? Now imagine that an attacker pads a path with a large number of '/'s at the beginning. This could result in future operations being performed on the file ``/''. If the program appends values in the belief that the result will be safe, the program may be exploitable. Or, the attacker could devise a long filename near the buffer length, so that attempts to append to the filename would silently fail to occur (or only partially occur in ways that may be exploitable).

When using statically-allocated buffers, you really need to consider the length of the source and destination arguments. Sanity checking the input and the resulting intermediate computation might deal with this, too.

Another alternative is to dynamically reallocate all strings instead of using fixed-size buffers. This general approach is recommended by the GNU programming guidelines, since it permits programs to handle arbitrarily-sized inputs (until they run out of memory). Of course, the major problem with dynamically allocated strings is that you may run out of memory. The memory may even be exhausted at some other point in the program than the portion where you're worried about buffer overflows; any memory allocation can fail. Also, since dynamic reallocation may cause memory to be inefficiently allocated, it is entirely possible to run out of memory even though technically there is enough virtual memory available to the program to continue. In addition, before running out of memory the program will probably use a great deal of virtual memory; this can easily result in ``thrashing'', a situation in which the computer spends all its time just shuttling information between the disk and memory (instead of doing useful work). This can have the effect of a denial of service attack. Some rational limits on input size can help here. In general, the program must be designed to fail safely when memory is exhausted if you use dynamically allocated strings.

6.2.3. strlcpy and strlcat

An alternative, being employed by OpenBSD, is the strlcpy(3) and strlcat(3) functions by Miller and de Raadt [Miller 1999]. This is a minimalist, statically-sized buffer approach that provides C string copying and concatenation with a different (and less error-prone) interface. Source and documentation of these functions are available under a newer BSD-style open source license at

First, here are their prototypes:
size_t strlcpy (char *dst, const char *src, size_t size);
 size_t strlcat (char *dst, const char *src, size_t size);
Both strlcpy and strlcat take the full size of the destination buffer as a parameter (not the maximum number of characters to be copied) and guarantee to NIL-terminate the result (as long as size is larger than 0). Remember that you should include a byte for NIL in the size.

The strlcpy function copies up to size-1 characters from the NUL-terminated string src to dst, NIL-terminating the result. The strlcat function appends the NIL-terminated string src to the end of dst. It will append at most size - strlen(dst) - 1 bytes, NIL-terminating the result.

One minor disadvantage of strlcpy(3) and strlcat(3) is that they are not, by default, installed in most Unix-like systems. In OpenBSD, they are part of <string.h>. This is not that difficult a problem; since they are small functions, you can even include them in your own program's source (at least as an option), and create a small separate package to load them. You can even use autoconf to handle this case automatically. If more programs use these functions, it won't be long before these are standard parts of Linux distributions and other Unix-like systems. Also, these functions have been recently added to the ``glib'' library (I submitted the patch to do this), so using recent versions of glib makes them available. In glib these functions are named g_strlcpy and g_strlcat (not strlcpy or strlcat) to be consistent with the glib library naming conventions.

Also, strlcat(3) has slightly varying semantics when the provided size is 0 or if there are no NIL characters in the destination string dst (inside the given number of characters). In OpenBSD, if the size is 0, then the destination string's length is considered 0. Also, if size is nonzero, but there are no NIL characters in the destination string (in the size number of characters), then the length of the destination is considered equal to the size. These rules make handling strings without embedded NILs consistent. Unfortunately, at least Solaris doesn't (at this time) obey these rules, because they weren't specified in the original documentation. I've talked to Todd Miller, and he and I agree that the OpenBSD semantics are the correct ones (and that Solaris is incorrect). The reasoning is simple: under no condition should strlcat or strlcpy ever examine characters in the destination outside of the range of size; such access might cause core dumps (from accessing out-of-range memory) and even hardware interactions (through memory-mapped I/O). Thus, given:
  a = strlcat ("Y", "123", 0);
The correct answer is 3 (0+3=3), but Solaris will claim the answer is 4 because it incorrectly looks at characters beyond the "size" length in the destination. For now, I suggest avoiding cases where the size is 0 or the destination has no NIL characters. Future versions of glib will hide this difference and always use the OpenBSD semantics.

6.2.4. libmib

One toolset for C that dynamically reallocates strings automatically is the ``libmib allocated string functions'' by Forrest J. Cavalier III, available at There are two variations of libmib; ``libmib-open'' appears to be clearly open source under its own X11-like license that permits modification and redistribution, but redistributions must choose a different name, however, the developer states that it ``may not be fully tested.'' To continuously get libmib-mature, you must pay for a subscription. The documentation is not open source, but it is freely available.

6.2.5. C++ std::string class

C++ developers can use the std::string class, which is built into the language. This is a dynamic approach, as the storage grows as necessary. However, it's important to note that if that class's data is turned into a ``char *'' (e.g., by using data() or c_str()), the possibilities of buffer overflow resurface, so you need to be careful when when using such methods. Note that c_str() always returns a NIL-terminated string, but data() may or may not (it's implementation dependent, and most implementations do not include the NIL terminator). Avoid using data(), and if you must use it, don't be dependent on its format.

Many C++ developers use other string libraries as well, such as those that come with other large libraries or even home-grown string libraries. With those libraries, be especially careful - many alternative C++ string classes include routines to automatically convert the class to a ``char *'' type. As a result, they can silently introduce buffer overflow vulnerabilities.

6.2.6. Libsafe

Arash Baratloo, Timothy Tsai, and Navjot Singh (of Lucent Technologies) have developed Libsafe, a wrapper of several library functions known to be vulnerable to stack smashing attacks. This wrapper (which they call a kind of ``middleware'') is a simple dynamically loaded library that contains modified versions of C library functions such as strcpy(3). These modified versions implement the original functionality, but in a manner that ensures that any buffer overflows are contained within the current stack frame. Their initial performance analysis suggests that this library's overhead is very small. Libsafe papers and source code are available at The Libsafe source code is available under the completely open source LGPL license.

Libsafe's approach appears somewhat useful. Libsafe should certainly be considered for inclusion by Linux distributors, and its approach is worth considering by others as well. For example, I know that the Mandrake distribution of Linux (version 7.1) includes it. However, as a software developer, Libsafe is a useful mechanism to support defense-in-depth but it does not really prevent buffer overflows. Here are several reasons why you shouldn't depend just on Libsafe during code development:

  • Libsafe only protects a small set of known functions with obvious buffer overflow issues. At the time of this writing, this list is significantly shorter than the list of functions in this book known to have this problem. It also won't protect against code you write yourself (e.g., in a while loop) that causes buffer overflows.

  • Even if libsafe is installed in a distribution, the way it is installed impacts its use. The documentation recommends setting LD_PRELOAD to cause libsafe's protections to be enabled, but the problem is that users can unset this environment variable... causing the protection to be disabled for programs they execute!

  • Libsafe only protects against buffer overflows of the stack onto the return address; you can still overrun the heap or other variables in that procedure's frame.

  • Unless you can be assured that all deployed platforms will use libsafe (or something like it), you'll have to protect your program as though it wasn't there.

  • LibSafe seems to assume that saved frame pointers are at the beginning of each stack frame. This isn't always true. Compilers (such as gcc) can optimize away things, and in particular the option "-fomit-frame-pointer" removes the information that libsafe seems to need. Thus, libsafe may fail to work for some programs.

The libsafe developers themselves acknowledge that software developers shouldn't just depend on libsafe. In their words:

It is generally accepted that the best solution to buffer overflow attacks is to fix the defective programs. However, fixing defective programs requires knowing that a particular program is defective. The true benefit of using libsafe and other alternative security measures is protection against future attacks on programs that are not yet known to be vulnerable.

6.2.7. Other Libraries

The glib (not glibc) library is a widely-available open source library that provides a number of useful functions for C programmers. GTK+ and GNOME both use glib, for example. As I noted earlier, in glib version 1.3.2, g_strlcpy() and g_strlcat() have been added through a patch which I submitted. This should make it easier to portably use those functions once these later versions of glib become widely available. At this time I do not have an analysis showing definitively that the glib library functions protect against buffer overflows. However, many of the glib functions automatically allocate memory, and those functions automatically fail with no reasonable way to intercept the failure (e.g., to try something else instead). As a result, in many cases most glib functions cannot be used in most secure programs. The GNOME guidelines recommend using functions such as g_strdup_printf(), which is fine as long as it's okay if your program immediately crashes if an out-of-memory condition occurs. However, if you can't accept this, then using such routines isn't appropriate.

6.3. Compilation Solutions in C/C++

A completely different approach is to use compilation methods that perform bounds-checking (see [Sitaker 1999] for a list). In my opinion, such tools are very useful in having multiple layers of defense, but it's not wise to use this technique as your sole defense. There are at least two reasons for this. First of all, such tools generally only provide a partial defense against buffer overflows (and the ``complete'' defenses are generally 12-30 times slower); C and C++ were simply not designed to protect against buffer overflows. Second of all, for open source programs you cannot be certain what tools will be used to compile the program; using the default ``normal'' compiler for a given system might suddenly open security flaws.

One of the more useful tools is ``StackGuard'', a modification of the standard GNU C compiler gcc. StackGuard works by inserting a ``guard'' value (called a ``canary'') in front of the return address; if a buffer overflow overwrites the return address, the canary's value (hopefully) changes and the system detects this before using it. This is quite valuable, but note that this does not protect against buffer overflows overwriting other values (which they may still be able to use to attack a system). There is work to extend StackGuard to be able to add canaries to other data items, called ``PointGuard''. PointGuard will automatically protect certain values (e.g., function pointers and longjump buffers). However, protecting other variable types using PointGuard requires specific programmer intervention (the programmer has to identify which data values must be protected with canaries). This can be valuable, but it's easy to accidentally omit protection for a data value you didn't think needed protection - but needs it anyway. More information on StackGuard, PointGuard, and other alternatives is in Cowan [1999].

IBM has developed a stack protection system called ProPolice based on the ideas of StackGuard. IBM doesn't include the ProPolice in its current website - it's just called a "GCC extension for protecting applications from stack-smashing attacks." Like StackGuard, ProPolice is a GCC (Gnu Compiler Collection) extension for protecting applications from stack-smashing attacks. Applications written in C are protected by automatically inserting protection code into an application at compilation time. ProPolice is slightly different than StackGuard, however, by adding three features: (1) reordering local variables to place buffers after pointers (to avoid the corruption of pointers that could be used to further corrupt arbitrary memory locations), (2) copying pointers in function arguments to an area preceding local variable buffers (to prevent the corruption of pointers that could be used to further corrupt arbitrary memory locations), and (3) omitting instrumentation code from some functions (it basically assumes that only character arrays are dangerous; while this isn't strictly true, it's mostly true, and as a result ProPolice has better performance while retaining most of its protective capabilities). The IBM website includes information for how to build Red Hat Linux and FreeBSD with this protection; OpenBSD has already added ProPolice to their base system. I think this is extremely promising, and I hope to see this capability included in future versions of gcc and used in various distributions. In fact, I think this kind of capability should be the default - this would mean that the largest single class of attacks would no longer enable attackers to take control in most cases.

As a related issue, in Linux you could modify the Linux kernel so that the stack segment is not executable; such a patch to Linux does exist (see Solar Designer's patch, which includes this, at However, as of this writing this is not built into the Linux kernel. Part of the rationale is that this is less protection than it seems; attackers can simply force the system to call other ``interesting'' locations already in the program (e.g., in its library, the heap, or static data segments). Also, sometimes Linux does require executable code in the stack, e.g., to implement signals and to implement GCC ``trampolines''. Solar Designer's patch does handle these cases, but this does complicate the patch. Personally, I'd like to see this merged into the main Linux distribution, since it does make attacks somewhat more difficult and it defends against a range of existing attacks. However, I agree with Linus Torvalds and others that this does not add the amount of protection it would appear to and can be circumvented with relative ease. You can read Linus Torvalds' explanation for not including this support at

In short, it's better to work first on developing a correct program that defends itself against buffer overflows. Then, after you've done this, by all means use techniques and tools like StackGuard as an additional safety net. If you've worked hard to eliminate buffer overflows in the code itself, then StackGuard (and tools like it) are are likely to be more effective because there will be fewer ``chinks in the armor'' that StackGuard will be called on to protect.

6.4. Other Languages

The problem of buffer overflows is an excellent argument for using other programming languages such as Perl, Python, Java, and Ada95. After all, nearly all other programming languages used today (other than assembly language) protect against buffer overflows. Using those other languages does not eliminate all problems, of course; in particular see the discussion in Section 8.3 regarding the NIL character. There is also the problem of ensuring that those other languages' infrastructure (e.g., run-time library) is available and secured. Still, you should certainly consider using other programming languages when developing secure programs to protect against buffer overflows.

Chapter 7. Structure Program Internals and Approach


Like a city whose walls are broken down is a man who lacks self-control.

 Proverbs 25:28 (NIV)

7.1. Follow Good Software Engineering Principles for Secure Programs

Saltzer [1974] and later Saltzer and Schroeder [1975] list the following principles of the design of secure protection systems, which are still valid:

  • Least privilege. Each user and program should operate using the fewest privileges possible. This principle limits the damage from an accident, error, or attack. It also reduces the number of potential interactions among privileged programs, so unintentional, unwanted, or improper uses of privilege are less likely to occur. This idea can be extended to the internals of a program: only the smallest portion of the program which needs those privileges should have them. See Section 7.4 for more about how to do this.

  • Economy of mechanism/Simplicity. The protection system's design should be simple and small as possible. In their words, ``techniques such as line-by-line inspection of software and physical examination of hardware that implements protection mechanisms are necessary. For such techniques to be successful, a small and simple design is essential.'' This is sometimes described as the ``KISS'' principle (``keep it simple, stupid'').

  • Open design. The protection mechanism must not depend on attacker ignorance. Instead, the mechanism should be public, depending on the secrecy of relatively few (and easily changeable) items like passwords or private keys. An open design makes extensive public scrutiny possible, and it also makes it possible for users to convince themselves that the system about to be used is adequate. Frankly, it isn't realistic to try to maintain secrecy for a system that is widely distributed; decompilers and subverted hardware can quickly expose any ``secrets'' in an implementation. Bruce Schneier argues that smart engineers should ``demand open source code for anything related to security'', as well as ensuring that it receives widespread review and that any identified problems are fixed [Schneier 1999].

  • Complete mediation. Every access attempt must be checked; position the mechanism so it cannot be subverted. For example, in a client-server model, generally the server must do all access checking because users can build or modify their own clients. This is the point of all of Chapter 5, as well as Section 7.2.

  • Fail-safe defaults (e.g., permission-based approach). The default should be denial of service, and the protection scheme should then identify conditions under which access is permitted. See Section 7.7 and Section 7.9 for more.

  • Separation of privilege. Ideally, access to objects should depend on more than one condition, so that defeating one protection system won't enable complete access.

  • Least common mechanism. Minimize the amount and use of shared mechanisms (e.g. use of the /tmp or /var/tmp directories). Shared objects provide potentially dangerous channels for information flow and unintended interactions. See Section 7.10 for more information.

  • Psychological acceptability / Easy to use. The human interface must be designed for ease of use so users will routinely and automatically use the protection mechanisms correctly. Mistakes will be reduced if the security mechanisms closely match the user's mental image of his or her protection goals.

A good overview of various design principles for security is available in Peter Neumann's Principled Assuredly Trustworthy Composable Architectures.

7.2. Secure the Interface

Interfaces should be minimal (simple as possible), narrow (provide only the functions needed), and non-bypassable. Trust should be minimized. Consider limiting the data that the user can see.

7.3. Separate Data and Control

Any files you support should be designed to completely separate (passive) data from programs that are executed. Applications and data viewers may be used to display files developed externally, so in general don't allow them to accept programs (also known as ``scripts'' or ``macros''). The most dangerous kind is an auto-executing macro that executes when the application is loaded and/or when the data is initially displayed; from a security point-of-view this is generally a disaster waiting to happen.

If you truly must support programs downloaded remotely (e.g., to implement an existing standard), make sure that you have extremely strong control over what the macro can do (this is often called a ``sandbox''). Past experience has shown that real sandboxes are hard to implement correctly. In fact, I can't remember a single widely-used sandbox that hasn't been repeatedly exploited (yes, that includes Java). If possible, at least have the programs stored in a separate file, so that it's easier to block them out when another sandbox flaw has been found but not yet fixed. Storing them separately also makes it easier to reuse code and to cache it when helpful.

7.4. Minimize Privileges

As noted earlier, it is an important general principle that programs have the minimal amount of privileges necessary to do its job (this is termed ``least privilege''). That way, if the program is broken, its damage is limited. The most extreme example is to simply not write a secure program at all - if this can be done, it usually should be. For example, don't make your program setuid or setgid if you can; just make it an ordinary program, and require the administrator to log in as such before running it.

In Linux and Unix, the primary determiner of a process' privileges is the set of id's associated with it: each process has a real, effective and saved id for both the user and group (a few very old Unixes don't have a ``saved'' id). Linux also has, as a special extension, a separate filesystem UID and GID for each process. Manipulating these values is critical to keeping privileges minimized, and there are several ways to minimize them (discussed below). You can also use chroot(2) to minimize the files visible to a program, though using chroot() can be difficult to use correctly. There are a few other values determining privilege in Linux and Unix, for example, POSIX capabilities (supported by Linux 2.2 and greater, and by some other Unix-like systems).

7.4.1. Minimize the Privileges Granted

Perhaps the most effective technique is to simply minimize the highest privilege granted. In particular, avoid granting a program root privilege if possible. Don't make a program setuid root if it only needs access to a small set of files; consider creating separate user or group accounts for different function.

A common technique is to create a special group, change a file's group ownership to that group, and then make the program setgid to that group. It's better to make a program setgid instead of setuid where you can, since group membership grants fewer rights (in particular, it does not grant the right to change file permissions).

This is commonly done for game high scores. Games are usually setgid games, the score files are owned by the group games, and the programs themselves and their configuration files are owned by someone else (say root). Thus, breaking into a game allows the perpetrator to change high scores but doesn't grant the privilege to change the game's executable or configuration file. The latter is important; if an attacker could change a game's executable or its configuration files (which might control what the executable runs), then they might be able to gain control of a user who ran the game.

If creating a new group isn't sufficient, consider creating a new pseudouser (really, a special role) to manage a set of resources - often a new pseudogroup (again, a special role) is also created just to run a program. Web servers typically do this; often web servers are set up with a special user (``nobody'') so that they can be isolated from other users. Indeed, web servers are instructive here: web servers typically need root privileges to start up (so they can attach to port 80), but once started they usually shed all their privileges and run as the user ``nobody''. However, don't use the ``nobody'' account (unless you're writing a webserver); instead, create your own pseudouser or new group. The purpose of this approach is to isolate different programs, processes, and data from each other, by exploiting the operating system's ability to keep users and groups separate. If different programs shared the same account, then breaking into one program would also grant privileges to the other. Usually the pseudouser should not own the programs it runs; that way, an attack who breaks into the account cannot change the program it runs. By isolating different parts of the system into running separate users and groups, breaking one part will not necessarily break the whole system's security.

If you're using a database system (say, by calling its query interface), limit the rights of the database user that the application uses. For example, don't give that user access to all of the system stored procedures if that user only needs access to a handful of user-defined ones. Do everything you can inside stored procedures. That way, even if someone does manage to force arbitrary strings into the query, the damage that can be done is limited. If you must directly pass a regular SQL query with client supplied data (and you usually shouldn't), wrap it in something that limits its activities (e.g., sp_sqlexec). (My thanks to SPI Labs for these database system suggestions).

If you must give a program privileges usually reserved for root, consider using POSIX capabilities as soon as your program can minimize the privileges available to your program. POSIX capabilities are available in Linux 2.2 and in many other Unix-like systems. By calling cap_set_proc(3) or the Linux-specific capsetp(3) routines immediately after starting, you can permanently reduce the abilities of your program to just those abilities it actually needs. For example the network time daemon (ntpd) traditionally has run as root, because it needs to modify the current time. However, patches have been developed so ntpd only needs a single capability, CAP_SYS_TIME, so even if an attacker gains control over ntpd it's somewhat more difficult to exploit the program.

I say ``somewhat limited'' because, unless other steps are taken, retaining a privilege using POSIX capabilities requires that the process continue to have the root user id. Because many important files (configuration files, binaries, and so on) are owned by root, an attacker controlling a program with such limited capabilities can still modify key system files and gain full root-level privilege. A Linux kernel extension (available in versions 2.4.X and 2.2.19+) provides a better way to limit the available privileges: a program can start as root (with all POSIX capabilities), prune its capabilities down to just what it needs, call prctl(PR_SET_KEEPCAPS,1), and then use setuid() to change to a non-root process. The PR_SET_KEEPCAPS setting marks a process so that when a process does a setuid to a nonzero value, the capabilities aren't cleared (normally they are cleared). This process setting is cleared on exec(). However, note that PR_SET_KEEPCAPS is a Linux-unique extension for newer versions of the linux kernel.

One tool you can use to simplify minimizing granted privileges is the ``compartment'' tool developed by SuSE. This tool, which only works on Linux, sets the filesystem root, uid, gid, and/or the capability set, then runs the given program. This is particularly handy for running some other program without modifying it. Here's the syntax of version 0.5:
 Syntax: compartment [options] /full/path/to/program
   --chroot path   chroot to path
   --user user     change UID to this user
   --group group   change GID to this group
   --init program  execute this program before doing anything
   --cap capset    set capset name. You can specify several
   --verbose       be verbose
   --quiet         do no logging (to syslog)

Thus, you could start a more secure anonymous ftp server using:
  compartment --chroot /home/ftp --cap CAP_NET_BIND_SERVICE anon-ftpd

At the time of this writing, the tool is immature and not available on typical Linux distributions, but this may quickly change. You can download the program via A similar tool is dreamland; you can that at

Note that not all Unix-like systems, implement POSIX capabilities, and PR_SET_KEEPCAPS is currently a Linux-only extension. Thus, these approaches limit portability. However, if you use it merely as an optional safeguard only where it's available, using this approach will not really limit portability. Also, while the Linux kernel version 2.2 and greater includes the low-level calls, the C-level libraries to make their use easy are not installed on some Linux distributions, slightly complicating their use in applications. For more information on Linux's implementation of POSIX capabilities, see

FreeBSD has the jail() function for limiting privileges; see the jail documentation for more information. There are a number of specialized tools and extensions for limiting privileges; see Section 3.10.

7.4.2. Minimize the Time the Privilege Can Be Used

As soon as possible, permanently give up privileges. Some Unix-like systems, including Linux, implement ``saved'' IDs which store the ``previous'' value. The simplest approach is to reset any supplemental groups if appropriate (e.g., using setgroups(2)), and then set the other id's twice to an untrusted id. In setuid/setgid programs, you should usually set the effective gid and uid to the real ones, in particular right after a fork(2), unless there's a good reason not to. Note that you have to change the gid first when dropping from root to another privilege or it won't work - once you drop root privileges, you won't be able to change much else. Note that in some systems, just setting the group isn't enough, if the process belongs to supplemental groups with privileges. For example, the ``rsync'' program didn't remove the supplementary groups when it changed its uid and gid, which created a potential exploit.

It's worth noting that there's a well-known related bug that uses POSIX capabilities to interfere with this minimization. This bug affects Linux kernel 2.2.0 through 2.2.15, and possibly a number of other Unix-like systems with POSIX capabilities. See Bugtraq id 1322 on for more information. Here is their summary:

POSIX "Capabilities" have recently been implemented in the Linux kernel. These "Capabilities" are an additional form of privilege control to enable more specific control over what privileged processes can do. Capabilities are implemented as three (fairly large) bitfields, which each bit representing a specific action a privileged process can perform. By setting specific bits, the actions of privileged processes can be controlled -- access can be granted for various functions only to the specific parts of a program that require them. It is a security measure. The problem is that capabilities are copied with fork() execs, meaning that if capabilities are modified by a parent process, they can be carried over. The way that this can be exploited is by setting all of the capabilities to zero (meaning, all of the bits are off) in each of the three bitfields and then executing a setuid program that attempts to drop privileges before executing code that could be dangerous if run as root, such as what sendmail does. When sendmail attempts to drop privileges using setuid(getuid()), it fails not having the capabilities required to do so in its bitfields and with no checks on its return value . It continues executing with superuser privileges, and can run a users .forward file as root leading to a complete compromise.

One approach, used by sendmail, is to attempt to do setuid(0) after a setuid(getuid()); normally this should fail. If it succeeds, the program should stop. For more information, see In the short term this might be a good idea in other programs, though clearly the better long-term approach is to upgrade the underlying system.

7.4.3. Minimize the Time the Privilege is Active

Use setuid(2), seteuid(2), setgroups(2), and related functions to ensure that the program only has these privileges active when necessary, and then temporarily deactivate the privilege when it's not in use. As noted above, you might want to ensure that these privileges are disabled while parsing user input, but more generally, only turn on privileges when they're actually needed.

Note that some buffer overflow attacks, if successful, can force a program to run arbitrary code, and that code could re-enable privileges that were temporarily dropped. Thus, there are many attacks that temporarily deactivating a privilege won't counter - it's always much better to completely drop privileges as soon as possible. There are many papers that describe how to do this, such as "Designing Shellcode Demystified". Some people even claim that ``seteuid() [is] considered harmful'' because of the many attacks it doesn't counter. Still, temporarily deactivating these permissions prevents a whole class of attacks, such as techniques to convince a program to write into a file that perhaps it didn't intend to write into. Since this technique prevents many attacks, it's worth doing if permanently dropping the privilege can't be done at that point in the program.

7.4.4. Minimize the Modules Granted the Privilege

If only a few modules are granted the privilege, then it's much easier to determine if they're secure. One way to do so is to have a single module use the privilege and then drop it, so that other modules called later cannot misuse the privilege. Another approach is to have separate commands in separate executables; one command might be a complex tool that can do a vast number of tasks for a privileged user (e.g., root), while the other tool is setuid but is a small, simple tool that only permits a small command subset (and does not trust its invoker). The small, simple tool checks to see if the input meets various criteria for acceptability, and then if it determines the input is acceptable, it passes the data on to the complex tool. Note that the small, simple tool must do a thorough job checking its inputs and limiting what it will pass along to the complex tool, or this can be a vulnerability. The communication could be via shell invocation, or any IPC mechanism. These approaches can even be layered several ways, for example, a complex user tool could call a simple setuid ``wrapping'' program (that checks its inputs for secure values) that then passes on information to another complex trusted tool.

This approach is the normal approach for developing GUI-based applications which requre privilege, but must be run by unprivileged users. The GUI portion is run as a normal unprivileged user process; that process then passes security-relevant requests on to another process that has the special privileges (and does not trust the first process, but instead limits the requests to whatever the user is allowed to do). Never develop a program that is privileged (e.g., using setuid) and also directly invokes a graphical toolkit: Graphical toolkits aren't designed to be used this way, and it would be extremely difficult to audit graphical toolkits in a way to make this possible. Fundamentally, graphical toolkits must be large, and it's extremely unwise to place so much faith in the perfection of that much code, so there is no point in trying to make them do what should never be done. Feel free to create a small setuid program that invokes two separate programs: one without privileges (but with the graphical interface), and one with privileges (and without an external interface). Or, create a small setuid program that can be invoked by the unprivileged GUI application. But never combine the two into a single process. For more about this, see the statement by Owen Taylor about GTK and setuid, discussing why GTK_MODULES is not a security hole.

Some applications can be best developed by dividing the problem into smaller, mutually untrusting programs. A simple way is divide up the problem into separate programs that do one thing (securely), using the filesystem and locking to prevent problems between them. If more complex interactions are needed, one approach is to fork into multiple processes, each of which has different privilege. Communications channels can be set up in a variety of ways; one way is to have a "master" process create communication channels (say unnamed pipes or unnamed sockets), then fork into different processes and have each process drop as many privileges as possible. If you're doing this, be sure to watch for deadlocks. Then use a simple protocol to allow the less trusted processes to request actions from the more trusted process(es), and ensure that the more trusted processes only support a limited set of requests. Setting user and group permissions so that no one else can even start up the sub-programs makes it harder to break into.

Some operating systems have the concept of multiple layers of trust in a single process, e.g., Multics' rings. Standard Unix and Linux don't have a way of separating multiple levels of trust by function inside a single process like this; a call to the kernel increases privileges, but otherwise a given process has a single level of trust. This is one area where technologies like Java 2, C# (which copies Java's approach), and Fluke (the basis of security-enhanced Linux) have an advantage. For example, Java 2 can specify fine-grained permissions such as the permission to only open a specific file. However, general-purpose operating systems do not typically have such abilities at this time; this may change in the near future. For more about Java, see Section 10.6.

7.4.5. Consider Using FSUID To Limit Privileges

Each Linux process has two Linux-unique state values called filesystem user id (FSUID) and filesystem group id (FSGID). These values are used when checking against the filesystem permissions. If you're building a program that operates as a file server for arbitrary users (like an NFS server), you might consider using these Linux extensions. To use them, while holding root privileges change just FSUID and FSGID before accessing files on behalf of a normal user. This extension is fairly useful, and provides a mechanism for limiting filesystem access rights without removing other (possibly necessary) rights. By only setting the FSUID (and not the EUID), a local user cannot send a signal to the process. Also, avoiding race conditions is much easier in this situation. However, a disadvantage of this approach is that these calls are not portable to other Unix-like systems.

7.4.6. Consider Using Chroot to Minimize Available Files

You can use chroot(2) to limit the files visible to your program. This requires carefully setting up a directory (called the ``chroot jail'') and correctly entering it. This can be a fairly effective technique for improving a program's security - it's hard to interfere with files you can't see. However, it depends on a whole bunch of assumptions, in particular, the program must lack root privileges, it must not have any way to get root privileges, and the chroot jail must be properly set up (e.g., be careful what you put inside the chroot jail, and make sure that users can never control its contents before calling chroot). I recommend using chroot(2) where it makes sense to do so, but don't depend on it alone; instead, make it part of a layered set of defenses. Here are a few notes about the use of chroot(2):

  • The program can still use non-filesystem objects that are shared across the entire machine (such as System V IPC objects and network sockets). It's best to also use separate pseudo-users and/or groups, because all Unix-like systems include the ability to isolate users; this will at least limit the damage a subverted program can do to other programs. Note that current most Unix-like systems (including Linux) won't isolate intentionally cooperating programs; if you're worried about malicious programs cooperating, you need to get a system that implements some sort of mandatory access control and/or limits covert channels.

  • Be sure to close any filesystem descriptors to outside files if you don't want them used later. In particular, don't have any descriptors open to directories outside the chroot jail, or set up a situation where such a descriptor could be given to it (e.g., via Unix sockets or an old implementation of /proc). If the program is given a descriptor to a directory outside the chroot jail, it could be used to escape out of the chroot jail.

  • The chroot jail has to be set up to be secure - it must never be controlled by a user and every file added must be carefully examined. Don't use a normal user's home directory, subdirectory, or other directory that can ever be controlled by a user as a chroot jail; use a separate directory directory specially set aside for the purpose. Using a directory controlled by a user is a disaster - for example, the user could create a ``lib'' directory containing a trojaned linker or libc (and could link a setuid root binary into that space, if the files you save don't use it). Place the absolute minimum number of files and directories there. Typically you'll have a /bin, /etc/, /lib, and maybe one or two others (e.g., /pub if it's an ftp server). Place in /bin only what you need to run after doing the chroot(); sometimes you need nothing at all (try to avoid placing a shell like /bin/sh there, though sometimes that can't be helped). You may need a /etc/passwd and /etc/group so file listings can show some correct names, but if so, try not to include the real system's values, and certainly replace all passwords with "*".

    In /lib, place only what you need; use ldd(1) to query each program in /bin to find out what it needs, and only include them. On Linux, you'll probably need a few basic libraries like, and not much else. Alternatively, recompile any necessary programs to be statically linked, so that they don't need dynamically loaded libraries at all.

    It's usually wiser to completely copy in all files, instead of making hard links; while this wastes some time and disk space, it makes it so that attacks on the chroot jail files do not automatically propagate into the regular system's files. Mounting a /proc filesystem, on systems where this is supported, is generally unwise. In fact, in very old versions of Linux (versions 2.0.x, at least up through 2.0.38) it's a known security flaw, since there are pseudo-directories in /proc that would permit a chroot'ed program to escape. Linux kernel 2.2 fixed this known problem, but there may be others; if possible, don't do it.

  • Chroot really isn't effective if the program can acquire root privilege. For example, the program could use calls like mknod(2) to create a device file that can view physical memory, and then use the resulting device file to modify kernel memory to give itself whatever privileges it desired. Another example of how a root program can break out of chroot is demonstrated at In this example, the program opens a file descriptor for the current directory, creates and chroots into a subdirectory, sets the current directory to the previously-opened current directory, repeatedly cd's up from the current directory (which since it is outside the current chroot succeeds in moving up to the real filesystem root), and then calls chroot on the result. By the time you read this, these weaknesses may have been plugged, but the reality is that root privilege has traditionally meant ``all privileges'' and it's hard to strip them away. It's better to assume that a program requiring continuous root privileges will only be mildly helped using chroot(). Of course, you may be able to break your program into parts, so that at least part of it can be in a chroot jail.

7.4.7. Consider Minimizing the Accessible Data

Consider minimizing the amount of data that can be accessed by the user. For example, in CGI scripts, place all data used by the CGI script outside of the document tree unless there is a reason the user needs to see the data directly. Some people have the false notion that, by not publicly providing a link, no one can access the data, but this is simply not true.

7.4.8. Consider Minimizing the Resources Available

Consider minimizing the computer resources available to a given process so that, even if it ``goes haywire,'' its damage can be limited. This is a fundamental technique for preventing a denial of service. For network servers, a common approach is to set up a separate process for each session, and for each process limit the amount of CPU time (et cetera) that session can use. That way, if an attacker makes a request that chews up memory or uses 100% of the CPU, the limits will kick in and prevent that single session from interfering with other tasks. Of course, an attacker can establish many sessions, but this at least raises the bar for an attack. See Section 3.6 for more information on how to set these limits (e.g., ulimit(1)).

7.5. Minimize the Functionality of a Component

In a related move, minimize the amount of functionality provided by your component. If it does several functions, consider breaking its implementation up into those smaller functions. That way, users who don't need some functions can disable just those portions. This is particularly important when a flaw is discovered - this way, users can disable just one component and still use the other parts.

7.6. Avoid Creating Setuid/Setgid Scripts

Many Unix-like systems, in particular Linux, simply ignore the setuid and setgid bits on scripts to avoid the race condition described earlier. Since support for setuid scripts varies on Unix-like systems, they're best avoided in new applications where possible. As a special case, Perl includes a special setup to support setuid Perl scripts, so using setuid and setgid is acceptable in Perl if you truly need this kind of functionality. If you need to support this kind of functionality in your own interpreter, examine how Perl does this. Otherwise, a simple approach is to ``wrap'' the script with a small setuid/setgid executable that creates a safe environment (e.g., clears and sets environment variables) and then calls the script (using the script's full path). Make sure that the script cannot be changed by an attacker! Shell scripting languages have additional problems, and really should not be setuid/setgid; see Section 10.4 for more information about this.

7.7. Configure Safely and Use Safe Defaults

Configuration is considered to currently be the number one security problem. Therefore, you should spend some effort to (1) make the initial installation secure, and (2) make it easy to reconfigure the system while keeping it secure.

Never have the installation routines install a working ``default'' password. If you need to install new ``users'', that's fine - just set them up with an impossible password, leaving time for administrators to set the password (and leaving the system secure before the password is set). Administrators will probably install hundreds of packages and almost certainly forget to set the password - it's likely they won't even know to set it, if you create a default password.

A program should have the most restrictive access policy until the administrator has a chance to configure it. Please don't create ``sample'' working users or ``allow access to all'' configurations as the starting configuration; many users just ``install everything'' (installing all available services) and never get around to configuring many services. In some cases the program may be able to determine that a more generous policy is reasonable by depending on the existing authentication system, for example, an ftp server could legitimately determine that a user who can log into a user's directory should be allowed to access that user's files. Be careful with such assumptions, however.

Have installation scripts install a program as safely as possible. By default, install all files as owned by root or some other system user and make them unwriteable by others; this prevents non-root users from installing viruses. Indeed, it's best to make them unreadable by all but the trusted user. Allow non-root installation where possible as well, so that users without root privileges and administrators who do not fully trust the installer can still use the program.

When installing, check to make sure that any assumptions necessary for security are true. Some library routines are not safe on some platforms; see the discussion of this in Section 8.1. If you know which platforms your application will run on, you need not check their specific attributes, but in that case you should check to make sure that the program is being installed on only one of those platforms. Otherwise, you should require a manual override to install the program, because you don't know if the result will be secure.

Try to make configuration as easy and clear as possible, including post-installation configuration. Make using the ``secure'' approach as easy as possible, or many users will use an insecure approach without understanding the risks. On Linux, take advantage of tools like linuxconf, so that users can easily configure their system using an existing infrastructure.

If there's a configuration language, the default should be to deny access until the user specifically grants it. Include many clear comments in the sample configuration file, if there is one, so the administrator understands what the configuration does.

7.8. Load Initialization Values Safely

Many programs read an initialization file to allow their defaults to be configured. You must ensure that an attacker can't change which initialization file is used, nor create or modify that file. Often you should not use the current directory as a source of this information, since if the program is used as an editor or browser, the user may be viewing the directory controlled by someone else. Instead, if the program is a typical user application, you should load any user defaults from a hidden file or directory contained in the user's home directory. If the program is setuid/setgid, don't read any file controlled by the user unless you carefully filter it as an untrusted (potentially hostile) input. Trusted configuration values should be loaded from somewhere else entirely (typically from a file in /etc).

7.9. Fail Safe

A secure program should always ``fail safe'', that is, it should be designed so that if the program does fail, the safest result should occur. For security-critical programs, that usually means that if some sort of misbehavior is detected (malformed input, reaching a ``can't get here'' state, and so on), then the program should immediately deny service and stop processing that request. Don't try to ``figure out what the user wanted'': just deny the service. Sometimes this can decrease reliability or useability (from a user's perspective), but it increases security. There are a few cases where this might not be desired (e.g., where denial of service is much worse than loss of confidentiality or integrity), but such cases are quite rare.

Note that I recommend ``stop processing the request'', not ``fail altogether''. In particular, most servers should not completely halt when given malformed input, because that creates a trivial opportunity for a denial of service attack (the attacker just sends garbage bits to prevent you from using the service). Sometimes taking the whole server down is necessary, in particular, reaching some ``can't get here'' states may signal a problem so drastic that continuing is unwise.

Consider carefully what error message you send back when a failure is detected. if you send nothing back, it may be hard to diagnose problems, but sending back too much information may unintentionally aid an attacker. Usually the best approach is to reply with ``access denied'' or ``miscellaneous error encountered'' and then write more detailed information to an audit log (where you can have more control over who sees the information).

7.10. Avoid Race Conditions

A ``race condition'' can be defined as ``Anomalous behavior due to unexpected critical dependence on the relative timing of events'' [FOLDOC]. Race conditions generally involve one or more processes accessing a shared resource (such a file or variable), where this multiple access has not been properly controlled.

In general, processes do not execute atomically; another process may interrupt it between essentially any two instructions. If a secure program's process is not prepared for these interruptions, another process may be able to interfere with the secure program's process. Any pair of operations in a secure program must still work correctly if arbitrary amounts of another process's code is executed between them.

Race condition problems can be notionally divided into two categories:

  • Interference caused by untrusted processes. Some security taxonomies call this problem a ``sequence'' or ``non-atomic'' condition. These are conditions caused by processes running other, different programs, which ``slip in'' other actions between steps of the secure program. These other programs might be invoked by an attacker specifically to cause the problem. This book will call these sequencing problems.

  • Interference caused by trusted processes (from the secure program's point of view). Some taxonomies call these deadlock, livelock, or locking failure conditions. These are conditions caused by processes running the ``same'' program. Since these different processes may have the ``same'' privileges, if not properly controlled they may be able to interfere with each other in a way other programs can't. Sometimes this kind of interference can be exploited. This book will call these locking problems.

7.10.1. Sequencing (Non-Atomic) Problems

In general, you must check your code for any pair of operations that might fail if arbitrary code is executed between them.

Note that loading and saving a shared variable are usually implemented as separate operations and are not atomic. This means that an ``increment variable'' operation is usually converted into loading, incrementing, and saving operation, so if the variable memory is shared the other process may interfere with the incrementing.

Secure programs must determine if a request should be granted, and if so, act on that request. There must be no way for an untrusted user to change anything used in this determination before the program acts on it. This kind of race condition is sometimes termed a ``time of check - time of use'' (TOCTOU) race condition. Atomic Actions in the Filesystem

The problem of failing to perform atomic actions repeatedly comes up in the filesystem. In general, the filesystem is a shared resource used by many programs, and some programs may interfere with its use by other programs. Secure programs should generally avoid using access(2) to determine if a request should be granted, followed later by open(2), because users may be able to move files around between these calls, possibly creating symbolic links or files of their own choosing instead. A secure program should instead set its effective id or filesystem id, then make the open call directly. It's possible to use access(2) securely, but only when a user cannot affect the file or any directory along its path from the filesystem root.

When creating a file, you should open it using the modes O_CREAT | O_EXCL and grant only very narrow permissions (only to the current user); you'll also need to prepare for having the open fail. If you need to be able to open the file (e.g,. to prevent a denial-of-service), you'll need to repetitively (1) create a ``random'' filename, (2) open the file as noted, and (3) stop repeating when the open succeeds.

Ordinary programs can become security weaknesses if they don't create files properly. For example, the ``joe'' text editor had a weakness called the ``DEADJOE'' symlink vulnerability. When joe was exited in a nonstandard way (such as a system crash, closing an xterm, or a network connection going down), joe would unconditionally append its open buffers to the file "DEADJOE". This could be exploited by the creation of DEADJOE symlinks in directories where root would normally use joe. In this way, joe could be used to append garbage to potentially-sensitive files, resulting in a denial of service and/or unintentional access.

As another example, when performing a series of operations on a file's meta-information (such as changing its owner, stat-ing the file, or changing its permission bits), first open the file and then use the operations on open files. This means use the fchown( ), fstat( ), or fchmod( ) system calls, instead of the functions taking filenames such as chown(), chgrp(), and chmod(). Doing so will prevent the file from being replaced while your program is running (a possible race condition). For example, if you close a file and then use chmod() to change its permissions, an attacker may be able to move or remove the file between those two steps and create a symbolic link to another file (say /etc/passwd). Other interesting files include /dev/zero, which can provide an infinitely-long data stream of input to a program; if an attacker can ``switch'' the file midstream, the results can be dangerous.

But even this gets complicated - when creating files, you must give them as a minimal set of rights as possible, and then change the rights to be more expansive if you desire. Generally, this means you need to use umask and/or open's parameters to limit initial access to just the user and user group. For example, if you create a file that is initially world-readable, then try to turn off the ``world readable'' bit, an attacker could try to open the file while the permission bits said this was okay. On most Unix-like systems, permissions are only checked on open, so this would result in an attacker having more privileges than intended.

In general, if multiple users can write to a directory in a Unix-like system, you'd better have the ``sticky'' bit set on that directory, and sticky directories had better be implemented. It's much better to completely avoid the problem, however, and create directories that only a trusted special process can access (and then implement that carefully). The traditional Unix temporary directories (/tmp and /var/tmp) are usually implemented as ``sticky'' directories, and all sorts of security problems can still surface, as we'll see next. Temporary Files

This issue of correctly performing atomic operations particularly comes up when creating temporary files. Temporary files in Unix-like systems are traditionally created in the /tmp or /var/tmp directories, which are shared by all users. A common trick by attackers is to create symbolic links in the temporary directory to some other file (e.g., /etc/passwd) while your secure program is running. The attacker's goal is to create a situation where the secure program determines that a given filename doesn't exist, the attacker then creates the symbolic link to another file, and then the secure program performs some operation (but now it actually opened an unintended file). Often important files can be clobbered or modified this way. There are many variations to this attack, such as creating normal files, all based on the idea that the attacker can create (or sometimes otherwise access) file system objects in the same directory used by the secure program for temporary files.

Michal Zalewski exposed in 2002 another serious problem with temporary directories involving automatic cleaning of temporary directories. For more information, see his posting to Bugtraq dated December 20, 2002, (subject "[RAZOR] Problems with mkstemp()"). Basically, Zalewski notes that it's a common practice to have a program automatically sweep temporary directories like /tmp and /var/tmp and remove "old" files that have not been accessed for a while (e.g., several days). Such programs are sometimes called "tmp cleaners" (pronounced "temp cleaners"). Possibly the most common tmp cleaner is "tmpwatch" by Erik Troan and Preston Brown of Red Hat Software; another common one is 'stmpclean' by Stanislav Shalunov; many administrators roll their own as well. Unfortunately, the existance of tmp cleaners creates an opportunity for new security-critical race conditions; an attacker may be able to arrange things so that the tmp cleaner interferes with the secure program. For example, an attacker could create an "old" file, arrange for the tmp cleaner to plan to delete the file, delete the file himself, and run a secure program that creates the same file - now the tmp cleaner will delete the secure program's file! Or, imagine that a secure program can have long delays after using the file (e.g., a setuid program stopped with SIGSTOP and resumed after many days with SIGCONT, or simply intentionally creating a lot of work). If the temporary file isn't used for long enough, its temporary files are likely to be removed by the tmp cleaner.

The general problem when creating files in these shared directories is that you must guarantee that the filename you plan to use doesn't already exist at time of creation, and atomically create the file. Checking ``before'' you create the file doesn't work, because after the check occurs, but before creation, another process can create that file with that filename. Using an ``unpredictable'' or ``unique'' filename doesn't work in general, because another process can often repeatedly guess until it succeeds. Once you create the file atomically, you must alway use the returned file descriptor (or file stream, if created from the file descriptor using routines like fdopen()). You must never re-open the file, or use any operations that use the filename as a parameter - always use the file descriptor or associated stream. Otherwise, the tmpwatch race issues noted above will cause problems. You can't even create the file, close it, and re-open it, even if the permissions limit who can open it. Note that comparing the descriptor and a reopened file to verify inode numbers, creation times or file ownership is not sufficient - please refer to "Symlinks and Cryogenic Sleep" by Olaf Kirch.

Fundamentally, to create a temporary file in a shared (sticky) directory, you must repetitively: (1) create a ``random'' filename, (2) open it using O_CREAT | O_EXCL and very narrow permissions (which atomically creates the file and fails if it's not created), and (3) stop repeating when the open succeeds.

According to the 1997 ``Single Unix Specification'', the preferred method for creating an arbitrary temporary file (using the C interface) is tmpfile(3). The tmpfile(3) function creates a temporary file and opens a corresponding stream, returning that stream (or NULL if it didn't). Unfortunately, the specification doesn't make any guarantees that the file will be created securely. In earlier versions of this book, I stated that I was concerned because I could not assure myself that all implementations do this securely. I've since found that older System V systems have an insecure implementation of tmpfile(3) (as well as insecure implementations of tmpnam(3) and tempnam(3)), so on at least some systems it's absolutely useless. Library implementations of tmpfile(3) should securely create such files, of course, but users don't always realize that their system libraries have this security flaw, and sometimes they can't do anything about it.

Kris Kennaway recommends using mkstemp(3) for making temporary files in general. His rationale is that you should use well-known library functions to perform this task instead of rolling your own functions, and that this function has well-known semantics. This is certainly a reasonable position. I would add that, if you use mkstemp(3), be sure to use umask(2) to limit the resulting temporary file permissions to only the owner. This is because some implementations of mkstemp(3) (basically older ones) make such files readable and writable by all, creating a condition in which an attacker can read or write private data in this directory. A minor nuisance is that mkstemp(3) doesn't directly support the environment variables TMP or TMPDIR (as discussed below), so if you want to support them you have to add code to do so. Here's a program in C that demonstrates how to use mkstemp(3) for this purpose, both directly and when adding support for TMP and TMPDIR:
#include <stdio.h>
 #include <stdlib.h>
 #include <sys/types.h>
 #include <sys/stat.h>
 void failure(msg) {
  fprintf(stderr, "%s\n", msg);
  * Given a "pattern" for a temporary filename
  * (starting with the directory location and ending in XXXXXX),
  * create the file and return it.
  * This routines unlinks the file, so normally it won't appear in
  * a directory listing.
  * The pattern will be changed to show the final filename.
 FILE *create_tempfile(char *temp_filename_pattern)
  int temp_fd;
  mode_t old_mode;
  FILE *temp_file;
  old_mode = umask(077);  /* Create file with restrictive permissions */
  temp_fd = mkstemp(temp_filename_pattern);
  (void) umask(old_mode);
  if (temp_fd == -1) {
    failure("Couldn't open temporary file");
  if (!(temp_file = fdopen(temp_fd, "w+b"))) {
    failure("Couldn't create temporary file's file descriptor");
  if (unlink(temp_filename_pattern) == -1) {
    failure("Couldn't unlink temporary file");
  return temp_file;
  * Given a "tag" (a relative filename ending in XXXXXX),
  * create a temporary file using the tag.  The file will be created
  * in the directory specified in the environment variables
  * TMPDIR or TMP, if defined and we aren't setuid/setgid, otherwise
  * it will be created in /tmp.  Note that root (and su'd to root)
  * _will_ use TMPDIR or TMP, if defined.
 FILE *smart_create_tempfile(char *tag)
  char *tmpdir = NULL;
  char *pattern;
  FILE *result;
  if ((getuid()==geteuid()) && (getgid()==getegid())) {
    if (! ((tmpdir=getenv("TMPDIR")))) {
  if (!tmpdir) {tmpdir = "/tmp";}
  pattern = malloc(strlen(tmpdir)+strlen(tag)+2);
  if (!pattern) {
    failure("Could not malloc tempfile pattern");
  strcpy(pattern, tmpdir);
  strcat(pattern, "/");
  strcat(pattern, tag);
  result = create_tempfile(pattern);
  return result;
 main() {
  int c;
  FILE *demo_temp_file1;
  FILE *demo_temp_file2;
  char demo_temp_filename1[] = "/tmp/demoXXXXXX";
  char demo_temp_filename2[] = "second-demoXXXXXX";
  demo_temp_file1 = create_tempfile(demo_temp_filename1);
  demo_temp_file2 = smart_create_tempfile(demo_temp_filename2);
  fprintf(demo_temp_file2, "This is a test.\n");
  printf("Printing temporary file contents:\n");
  while (  (c=fgetc(demo_temp_file2)) != EOF) {
  printf("Exiting; you'll notice that there are no temporary files on exit.\n");

Kennaway states that if you can't use mkstemp(3), then make yourself a directory using mkdtemp(3), which is protected from the outside world. However, as Michal Zalewski notes, this is a bad idea if there are tmp cleaners in use; instead, use a directory inside the user's HOME. Finally, if you really have to use the insecure mktemp(3), use lots of X's - he suggests 10 (if your libc allows it) so that the filename can't easily be guessed (using only 6 X's means that 5 are taken up by the PID, leaving only one random character and allowing an attacker to mount an easy race condition). Note that this is fundamentally insecure, so you should normally not do this. I add that you should avoid tmpnam(3) as well - some of its uses aren't reliable when threads are present, and it doesn't guarantee that it will work correctly after TMP_MAX uses (yet most practical uses must be inside a loop).

In general, you should avoid using the insecure functions such as mktemp(3) or tmpnam(3), unless you take specific measures to counter their insecurities or test for a secure library implementation as part of your installation routines. If you ever want to make a file in /tmp or a world-writable directory (or group-writable, if you don't trust the group) and don't want to use mk*temp() (e.g. you intend for the file to be predictably named), then always use the O_CREAT and O_EXCL flags to open() and check the return value. If you fail the open() call, then recover gracefully (e.g. exit).

The GNOME programming guidelines recommend the following C code when creating filesystem objects in shared (temporary) directories to securely open temporary files [Quintero 2000]:
 char *filename;
  int fd;
  do {
    filename = tempnam (NULL, "foo");
    fd = open (filename, O_CREAT | O_EXCL | O_TRUNC | O_RDWR, 0600);
    free (filename);
  } while (fd == -1);
Note that, although the insecure function tempnam(3) is being used, it is wrapped inside a loop using O_CREAT and O_EXCL to counteract its security weaknesses, so this use is okay. Note that you need to free() the filename. You should close() and unlink() the file after you are done. If you want to use the Standard C I/O library, you can use fdopen() with mode "w+b" to transform the file descriptor into a FILE *. Note that this approach won't work over NFS version 2 (v2) systems, because older NFS doesn't correctly support O_EXCL. Note that one minor disadvantage to this approach is that, since tempnam can be used insecurely, various compilers and security scanners may give you spurious warnings about its use. This isn't a problem with mkstemp(3).

If you need a temporary file in a shell script, you're probably best off using pipes, using a local directory (e.g., something inside the user's home directory), or in some cases using the current directory. That way, there's no sharing unless the user permits it. If you really want/need the temporary file to be in a shared directory like /tmp, do not use the traditional shell technique of using the process id in a template and just creating the file using normal operations like ">". Shell scripts can use "$$" to indicate the PID, but the PID can be easily determined or guessed by an attacker, who can then pre-create files or links with the same name. Thus the following "typical" shell script is unsafe:
   echo "This is a test" > /tmp/test$$  # DON'T DO THIS.

If you need a temporary file or directory in a shell script, and you want it in /tmp, a solution sometimes suggested is to use mktemp(1), which is intended for use in shell scripts (note that mktemp(1) and mktemp(3) are different things). However, as Michal Zalewski notes, this is insecure in many environments that run tmp cleaners; the problem is that when a privileged program sweeps through a temporary directory, it will probably expose a race condition. Even if this weren't true, I do not recommend using shell scripts that create temporary files in shared directories; creating such files in private directories or using pipes instead is generally preferable, even if you're sure your tmpwatch program is okay (or that you have no local users). If you must use mktemp(1), note that mktemp(1) takes a template, then creates a file or directory using O_EXCL and returns the resulting name; thus, mktemp(1) won't work on NFS version 2 filesystems. Here are some examples of correct use of mktemp(1) in Bourne shell scripts; these examples are straight from the mktemp(1) man page:
 # Simple use of mktemp(1), where the script should quit
  # if it can't get a safe temporary file.
  # Note that this will be INSECURE on many systems, since they use
  # tmpwatch-like programs that will erase "old" files and expose race
  # conditions.
    TMPFILE=`mktemp /tmp/$0.XXXXXX` || exit 1
    echo "program output" >> $TMPFILE
   # Simple example, if you want to catch the error:
    TMPFILE=`mktemp -q /tmp/$0.XXXXXX`
    if [ $? -ne 0 ]; then
       echo "$0: Can't create temp file, exiting..."
       exit 1

Perl programmers should use File::Temp, which tries to provide a cross-platform means of securely creating temporary files. However, read the documentation carefully on how to use it properly first; it includes interfaces to unsafe functions as well. I suggest explicitly setting its safe_level to HIGH; this will invoke additional security checks. The Perl 5.8 documentation of File::Temp is available on-line.

Don't reuse a temporary filename (i.e. remove and recreate it), no matter how you obtained the ``secure'' temporary filename in the first place. An attacker can observe the original filename and hijack it before you recreate it the second time. And of course, always use appropriate file permissions. For example, only allow world/group access if you need the world or a group to access the file, otherwise keep it mode 0600 (i.e., only the owner can read or write it).

Clean up after yourself, either by using an exit handler, or making use of UNIX filesystem semantics and unlink()ing the file immediately after creation so the directory entry goes away but the file itself remains accessible until the last file descriptor pointing to it is closed. You can then continue to access it within your program by passing around the file descriptor. Unlinking the file has a lot of advantages for code maintenance: the file is automatically deleted, no matter how your program crashes. It also decreases the likelihood that a maintainer will insecurely use the filename (they need to use the file descriptor instead). The one minor problem with immediate unlinking is that it makes it slightly harder for administrators to see how disk space is being used, since they can't simply look at the file system by name.

You might consider ensuring that your code for Unix-like systems respects the environment variables TMP or TMPDIR if the provider of these variable values is trusted. By doing so, you make it possible for users to move their temporary files into an unshared directory (and eliminating the problems discussed here), such as a subdirectory inside their home directory. Recent versions of Bastille can set these variables to reduce the sharing between users. Unfortunately, many users set TMP or TMPDIR to a shared directory (say /tmp), so your secure program must still correctly create temporary files even if these environment variables are set. This is one advantage of the GNOME approach, since at least on some systems tempnam(3) automatically uses TMPDIR, while the mkstemp(3) approach requires more code to do this. Please don't create yet more environment variables for temporary directories (such as TEMP), and in particular don't create a different environment name for each application (e.g., don't use "MYAPP_TEMP"). Doing so greatly complicates managing systems, and users wanting a special temporary directory for a specific application can just set the environment variable specially when running that particular application. Of course, if these environment variables might have been set by an untrusted source, you should ignore them - which you'll do anyway if you follow the advice in Section 5.2.3.

These techniques don't work if the temporary directory is remotely mounted using NFS version 2 (NFSv2), because NFSv2 doesn't properly support O_EXCL. See Section for more information. NFS version 3 and later properly support O_EXCL; the simple solution is to ensure that temporary directories are either local or, if mounted using NFS, mounted using NFS version 3 or later. There is a technique for safely creating temporary files on NFS v2, involving the use of link(2) and stat(2), but it's complex; see Section which has more information about this.

As an aside, it's worth noting that FreeBSD has recently changed the mk*temp() family to get rid of the PID component of the filename and replace the entire thing with base-62 encoded randomness. This drastically raises the number of possible temporary files for the "default" usage of 6 X's, meaning that even mktemp(3) with 6 X's is reasonably (probabilistically) secure against guessing, except under very frequent usage. However, if you also follow the guidance here, you'll eliminate the problem they're addressing.

Much of this information on temporary files was derived from Kris Kennaway's posting to Bugtraq about temporary files on December 15, 2000.

I should note that the Openwall Linux patch from includes an optional ``temporary file directory'' policy that counters many temporary file based attacks. The Linux Security Module (LSM) project includes an "owlsm" module that implements some of the OpenWall ideas, so Linux Kernels with LSM can quickly insert these rules into a running system. When enabled, it has two protections:

  • Hard links: Processes may not make hard links to files in certain cases. The OpenWall documentation states that "Processes may not make hard links to files they do not have write access to." In the LSM version, the rules are as follows: if both the process' uid and fsuid (usually the same as the euid) is is different from the linked-to-file's uid, the process uid is not root, and the process lacks the FOWNER capability, then the hard link is forbidden. The check against the process uid may be dropped someday (they are work-arounds for the atd(8) program), at which point the rules would be: if both the process' fsuid (usually the same as the euid) is is different from the linked-to-file's uid and and the process lacks the FOWNER capability, then the hard link is forbidden. In other words, you can only create hard links to files you own, unless you have the FOWNER capability.

  • Symbolic links (symlinks): Certain symlinks are not followed. The original OpenWall documentation states that "root processes may not follow symlinks that are not owned by root", but the actual rules (from looking at the code) are more complicated. In the LSM version, if the directory is sticky ("+t" mode, used in shared directories like /tmp), symlinks are not followed if the symlink was created by anyone other than either the owner of the directory or the current process' fsuid (which is usually the effective uid).

Many systems do not implement this openwall policy, so you can't depend on this in general protecting your system. However, I encourage using this policy on your own system, and please make sure that your application will work when this policy is in place.

7.10.2. Locking

There are often situations in which a program must ensure that it has exclusive rights to something (e.g., a file, a device, and/or existence of a particular server process). Any system which locks resources must deal with the standard problems of locks, namely, deadlocks (``deadly embraces''), livelocks, and releasing ``stuck'' locks if a program doesn't clean up its locks. A deadlock can occur if programs are stuck waiting for each other to release resources. For example, a deadlock would occur if process 1 locks resources A and waits for resource B, while process 2 locks resource B and waits for resource A. Many deadlocks can be prevented by simply requiring all processes that lock multiple resources to lock them in the same order (e.g., alphabetically by lock name). Using Files as Locks

On Unix-like systems resource locking has traditionally been done by creating a file to indicate a lock, because this is very portable. It also makes it easy to ``fix'' stuck locks, because an administrator can just look at the filesystem to see what locks have been set. Stuck locks can occur because the program failed to clean up after itself (e.g., it crashed or malfunctioned) or because the whole system crashed. Note that these are ``advisory'' (not ``mandatory'') locks - all processes needed the resource must cooperate to use these locks.

However, there are several traps to avoid. First, don't use the technique used by very old Unix C programs, which is calling creat() or its open() equivalent, the open() mode O_WRONLY | O_CREAT | O_TRUNC, with the file mode set to 0 (no permissions). For normal users on normal file systems, this works, but this approach fails to lock the file when the user has root privileges. Root can always perform this operation, even when the file already exists. In fact, old versions of Unix had this particular problem in the old editor ``ed'' -- the symptom was that occasionally portions of the password file would be placed in user's files [Rochkind 1985, 22]! Instead, if you're creating a lock for processes that are on the local filesystem, you should use open() with the flags O_WRONLY | O_CREAT | O_EXCL (and again, no permissions, so that other processes with the same owner won't get the lock). Note the use of O_EXCL, which is the official way to create ``exclusive'' files; this even works for root on a local filesystem. [Rochkind 1985, 27].

Second, if the lock file may be on an NFS-mounted filesystem, then you have the problem that NFS version 2 doesn't completely support normal file semantics. This can even be a problem for work that's supposed to be ``local'' to a client, since some clients don't have local disks and may have all files remotely mounted via NFS. The manual for open(2) explains how to handle things in this case (which also handles the case of root programs):

"... programs which rely on [the O_CREAT and O_EXCL flags of open(2) to work on filesystems accessed via NFS version 2] for performing locking tasks will contain a race condition. The solution for performing atomic file locking using a lockfile is to create a unique file on the same filesystem (e.g., incorporating hostname and pid), use link(2) to make a link to the lockfile and use stat(2) on the unique file to check if its link count has increased to 2. Do not use the return value of the link(2) call."

Obviously, this solution only works if all programs doing the locking are cooperating, and if all non-cooperating programs aren't allowed to interfere. In particular, the directories you're using for file locking must not have permissive file permissions for creating and removing files.

NFS version 3 added support for O_EXCL mode in open(2); see IETF RFC 1813, in particular the "EXCLUSIVE" value to the "mode" argument of "CREATE". Sadly, not everyone has switched to NFS version 3 or higher at the time of this writing, so you can't depend on this yet in portable programs. Still, in the long run there's hope that this issue will go away.

If you're locking a device or the existence of a process on a local machine, try to use standard conventions. I recommend using the Filesystem Hierarchy Standard (FHS); it is widely referenced by Linux systems, but it also tries to incorporate the ideas of other Unix-like systems. The FHS describes standard conventions for such locking files, including naming, placement, and standard contents of these files [FHS 1997]. If you just want to be sure that your server doesn't execute more than once on a given machine, you should usually create a process identifier as /var/run/ with the pid as its contents. In a similar vein, you should place lock files for things like device lock files in /var/lock. This approach has the minor disadvantage of leaving files hanging around if the program suddenly halts, but it's standard practice and that problem is easily handled by other system tools.

It's important that the programs which are cooperating using files to represent the locks use the same directory, not just the same directory name. This is an issue with networked systems: the FHS explicitly notes that /var/run and /var/lock are unshareable, while /var/mail is shareable. Thus, if you want the lock to work on a single machine, but not interfere with other machines, use unshareable directories like /var/run (e.g., you want to permit each machine to run its own server). However, if you want all machines sharing files in a network to obey the lock, you need to use a directory that they're sharing; /var/mail is one such location. See FHS section 2 for more information on this subject. Other Approaches to Locking

Of course, you need not use files to represent locks. Network servers often need not bother; the mere act of binding to a port acts as a kind of lock, since if there's an existing server bound to a given port, no other server will be able to bind to that port.

Another approach to locking is to use POSIX record locks, implemented through fcntl(2) as a ``discretionary lock''. These are discretionary, that is, using them requires the cooperation of the programs needing the locks (just as the approach to using files to represent locks does). There's a lot to recommend POSIX record locks: POSIX record locking is supported on nearly all Unix-like platforms (it's mandated by POSIX.1), it can lock portions of a file (not just a whole file), and it can handle the difference between read locks and write locks. Even more usefully, if a process dies, its locks are automatically removed, which is usually what is desired.

You can also use mandatory locks, which are based on System V's mandatory locking scheme. These only apply to files where the locked file's setgid bit is set, but the group execute bit is not set. Also, you must mount the filesystem to permit mandatory file locks. In this case, every read(2) and write(2) is checked for locking; while this is more thorough than advisory locks, it's also slower. Also, mandatory locks don't port as widely to other Unix-like systems (they're available on Linux and System V-based systems, but not necessarily on others). Note that processes with root privileges can be held up by a mandatory lock, too, making it possible that this could be the basis of a denial-of-service attack.

7.11. Trust Only Trustworthy Channels

In general, only trust information (input or results) from trustworthy channels. For example, the routines getlogin(3) and ttyname(3) return information that can be controlled by a local user, so don't trust them for security purposes.

In most computer networks (and certainly for the Internet at large), no unauthenticated transmission is trustworthy. For example, packets sent over the public Internet can be viewed and modified at any point along their path, and arbitrary new packets can be forged. These forged packets might include forged information about the sender (such as their machine (IP) address and port) or receiver. Therefore, don't use these values as your primary criteria for security decisions unless you can authenticate them (say using cryptography).

This means that, except under special circumstances, two old techniques for authenticating users in TCP/IP should often not be used as the sole authentication mechanism. One technique is to limit users to ``certain machines'' by checking the ``from'' machine address in a data packet; the other is to limit access by requiring that the sender use a ``trusted'' port number (a number less that 1024). The problem is that in many environments an attacker can forge these values.

In some environments, checking these values (e.g., the sending machine IP address and/or port) can have some value, so it's not a bad idea to support such checking as an option in a program. For example, if a system runs behind a firewall, the firewall can't be breached or circumvented, and the firewall stops external packets that claim to be from the inside, then you can claim that any packet saying it's from the inside really does. Note that you can't be sure the packet actually comes from the machine it claims it comes from - so you're only countering external threats, not internal threats. However, broken firewalls, alternative paths, and mobile code make even these assumptions suspect.

The problem is supporting untrustworthy information as the only way to authenticate someone. If you need a trustworthy channel over an untrusted network, in general you need some sort of cryptologic service (at the very least, a cryptologically safe hash). See Section 11.5 for more information on cryptographic algorithms and protocols. If you're implementing a standard and inherently insecure protocol (e.g., ftp and rlogin), provide safe defaults and document the assumptions clearly.

The Domain Name Server (DNS) is widely used on the Internet to maintain mappings between the names of computers and their IP (numeric) addresses. The technique called ``reverse DNS'' eliminates some simple spoofing attacks, and is useful for determining a host's name. However, this technique is not trustworthy for authentication decisions. The problem is that, in the end, a DNS request will be sent eventually to some remote system that may be controlled by an attacker. Therefore, treat DNS results as an input that needs validation and don't trust it for serious access control.

Arbitrary email (including the ``from'' value of addresses) can be forged as well. Using digital signatures is a method to thwart many such attacks. A more easily thwarted approach is to require emailing back and forth with special randomly-created values, but for low-value transactions such as signing onto a public mailing list this is usually acceptable.

Note that in any client/server model, including CGI, that the server must assume that the client (or someone interposing between the client and server) can modify any value. For example, so-called ``hidden fields'' and cookie values can be changed by the client before being received by CGI programs. These cannot be trusted unless special precautions are taken. For example, the hidden fields could be signed in a way the client cannot forge as long as the server checks the signature. The hidden fields could also be encrypted using a key only the trusted server could decrypt (this latter approach is the basic idea behind the Kerberos authentication system). InfoSec labs has further discussion about hidden fields and applying encryption at In general, you're better off keeping data you care about at the server end in a client/server model. In the same vein, don't depend on HTTP_REFERER for authentication in a CGI program, because this is sent by the user's browser (not the web server).

This issue applies to data referencing other data, too. For example, HTML or XML allow you to include by reference other files (e.g., DTDs and style sheets) that may be stored remotely. However, those external references could be modified so that users see a very different document than intended; a style sheet could be modified to ``white out'' words at critical locations, deface its appearance, or insert new text. External DTDs could be modified to prevent use of the document (by adding declarations that break validation) or insert different text into documents [St. Laurent 2000].

7.12. Set up a Trusted Path

The counterpart to needing trustworthy channels (see Section 7.11) is assuring users that they really are working with the program or system they intended to use.

The traditional example is a ``fake login'' program. If a program is written to look like the login screen of a system, then it can be left running. When users try to log in, the fake login program can then capture user passwords for later use.

A solution to this problem is a ``trusted path.'' A trusted path is simply some mechanism that provides confidence that the user is communicating with what the user intended to communicate with, ensuring that attackers can't intercept or modify whatever information is being communicated.

If you're asking for a password, try to set up trusted path. Unfortunately, stock Linux distributions and many other Unixes don't have a trusted path even for their normal login sequence. One approach is to require pressing an unforgeable key before login, e.g., Windows NT/2000 uses ``control-alt-delete'' before logging in; since normal programs in Windows can't intercept this key pattern, this approach creates a trusted path. There's a Linux equivalent, termed the Secure Attention Key (SAK); it's recommended that this be mapped to ``control-alt-pause''. Unfortunately, at the time of this writing SAK is immature and not well-supported by Linux distributions. Another approach for implementing a trusted path locally is to control a separate display that only the login program can perform. For example, if only trusted programs could modify the keyboard lights (the LEDs showing Num Lock, Caps Lock, and Scroll Lock), then a login program could display a running pattern to indicate that it's the real login program. Unfortunately, since in current Linux normal users can change the LEDs, the LEDs can't currently be used to confirm a trusted path.

Sadly, the problem is much worse for network applications. Although setting up a trusted path is desirable for network applications, completely doing so is quite difficult. When sending a password over a network, at the very least encrypt the password between trusted endpoints. This will at least prevent eavesdropping of passwords by those not connected to the system, and at least make attacks harder to perform. If you're concerned about trusted path for the actual communication, make sure that the communication is encrypted and authenticated (or at least authenticated).

It turns out that this isn't enough to have a trusted path to networked applications, in particular for web-based applications. There are documented methods for fooling users of web browsers into thinking that they're at one place when they are really at another. For example, Felten [1997] discusses ``web spoofing'', where users believe they're viewing one web page when in fact all the web pages they view go through an attacker's site (who can then monitor all traffic and modify any data sent in either direction). This is accomplished by rewriting URL. The rewritten URLs can be made nearly invisible by using other technology (such as Javascript) to hide any possible evidence in the status line, location line, and so on. See their paper for more details. Another technique for hiding such URLs is exploiting rarely-used URL syntax, for example, the URL ``'' is actually a request to view ``'' (a potentially malevolent site) using the unusual username ``'. If the URL is long enough, the real material won't be displayed and users are unlikely to notice the exploit anyway. Yet another approach is to create sites with names deliberately similar to the ``real'' site - users may not know the difference. In all of these cases, simply encrypting the line doesn't help - the attacker can be quite content in encrypting data while completely controlling what's shown.

Countering these problems is more difficult; at this time I have no good technical solution for fully preventing ``fooled'' web users. I would encourage web browser developers to counter such ``fooling'', making it easier to spot. If it's critical that your users correctly connect to the correct site, have them use simple procedures to counter the threat. Examples include having them halt and restart their browser, and making sure that the web address is very simple and not normally misspelled (so misspelling it is unlikely). You might also want to gain ownership of some ``similar'' sounding DNS names, and search for other such DNS names and material to find attackers.

7.13. Use Internal Consistency-Checking Code

The program should check to ensure that its call arguments and basic state assumptions are valid. In C, macros such as assert(3) may be helpful in doing so.

7.14. Self-limit Resources

In network daemons, shed or limit excessive loads. Set limit values (using setrlimit(2)) to limit the resources that will be used. At the least, use setrlimit(2) to disable creation of ``core'' files. For example, by default Linux will create a core file that saves all program memory if the program fails abnormally, but such a file might include passwords or other sensitive data.

7.15. Prevent Cross-Site (XSS) Malicious Content

Some secure programs accept data from one untrusted user (the attacker) and pass that data on to a different user's application (the victim). If the secure program doesn't protect the victim, the victim's application (e.g., their web browser) may then process that data in a way harmful to the victim. This is a particularly common problem for web applications using HTML or XML, where the problem goes by several names including ``cross-site scripting'', ``malicious HTML tags'', and ``malicious content.'' This book will call this problem ``cross-site malicious content,'' since the problem isn't limited to scripts or HTML, and its cross-site nature is fundamental. Note that this problem isn't limited to web applications, but since this is a particular problem for them, the rest of this discussion will emphasize web applications. As will be shown in a moment, sometimes an attacker can cause a victim to send data from the victim to the secure program, so the secure program must protect the victim from himself.

7.15.1. Explanation of the Problem

Let's begin with a simple example. Some web applications are designed to permit HTML tags in data input from users that will later be posted to other readers (e.g., in a guestbook or ``reader comment'' area). If nothing is done to prevent it, these tags can be used by malicious users to attack other users by inserting scripts, Java references (including references to hostile applets), DHTML tags, early document endings (via </HTML>), absurd font size requests, and so on. This capability can be exploited for a wide range of effects, such as exposing SSL-encrypted connections, accessing restricted web sites via the client, violating domain-based security policies, making the web page unreadable, making the web page unpleasant to use (e.g., via annoying banners and offensive material), permit privacy intrusions (e.g., by inserting a web bug to learn exactly who reads a certain page), creating denial-of-service attacks (e.g., by creating an ``infinite'' number of windows), and even very destructive attacks (by inserting attacks on security vulnerabilities such as scripting languages or buffer overflows in browsers). By embedding malicious FORM tags at the right place, an intruder may even be able to trick users into revealing sensitive information (by modifying the behavior of an existing form). Or, by embedding scripts, an intruder can cause no end of problems. This is by no means an exhaustive list of problems, but hopefully this is enough to convince you that this is a serious problem.

Most ``discussion boards'' have already discovered this problem, and most already take steps to prevent it in text intended to be part of a multiperson discussion. Unfortunately, many web application developers don't realize that this is a much more general problem. Every data value that is sent from one user to another can potentially be a source for cross-site malicious posting, even if it's not an ``obvious'' case of an area where arbitrary HTML is expected. The malicious data can even be supplied by the user himself, since the user may have been fooled into supplying the data via another site. Here's an example (from CERT) of an HTML link that causes the user to send malicious data to another site:
  SRC='http://bad-site/badfile'></SCRIPT>"> Click here</A>

In short, a web application cannot accept input (including any form data) without checking, filtering, or encoding it. You can't even pass that data back to the same user in many cases in web applications, since another user may have surreptitiously supplied the data. Even if permitting such material won't hurt your system, it will enable your system to be a conduit of attacks to your users. Even worse, those attacks will appear to be coming from your system.

CERT describes the problem this way in their advisory:

A web site may inadvertently include malicious HTML tags or script in a dynamically generated page based on unvalidated input from untrustworthy sources (CERT Advisory CA-2000-02, Malicious HTML Tags Embedded in Client Web Requests).

More information from CERT about this is available at

7.15.2. Solutions to Cross-Site Malicious Content

Fundamentally, this means that all web application output impacted by any user must be filtered (so characters that can cause this problem are removed), encoded (so the characters that can cause this problem are encoded in a way to prevent the problem), or validated (to ensure that only ``safe'' data gets through). This includes all output derived from input such as URL parameters, form data, cookies, database queries, CORBA ORB results, and data from users stored in files. In many cases, filtering and validation should be done at the input, but encoding can be done during either input validation or output generation. If you're just passing the data through without analysis, it's probably better to encode the data on input (so it won't be forgotten). However, if your program processes the data, it can be easier to encode it on output instead. CERT recommends that filtering and encoding be done during data output; this isn't a bad idea, but there are many cases where it makes sense to do it at input instead. The critical issue is to make sure that you cover all cases for every output, which is not an easy thing to do regardless of approach.

Warning - in many cases these techniques can be subverted unless you've also gained control over the character encoding of the output. Otherwise, an attacker could use an ``unexpected'' character encoding to subvert the techniques discussed here. Thankfully, this isn't hard; gaining control over output character encoding is discussed in Section 9.5.

One minor defense, that's often worth doing, is the "HttpOnly" flag for cookies. Scripts that run in a web browser cannot access cookie values that have the HttpOnly flag set (they just get an empty value instead). This is currently implemented in Microsoft Internet Explorer, and I expect Mozilla/Netscape to implement this soon too. You should set HttpOnly on for any cookie you send, unless you have scripts that need the cookie, to counter certain kinds of cross-site scripting (XSS) attacks. However, the HttpOnly flag can be circumvented in a variety of ways, so using as your primary defense is inappropriate. Instead, it's a helpful secondary defense that may help save you in case your application is written incorrectly.

The first subsection below discusses how to identify special characters that need to be filtered, encoded, or validated. This is followed by subsections describing how to filter or encode these characters. There's no subsection discussing how to validate data in general, however, for input validation in general see Chapter 5, and if the input is straight HTML text or a URI, see Section 5.11. Also note that your web application can receive malicious cross-postings, so non-queries should forbid the GET protocol (see Section 5.12). Identifying Special Characters

Here are the special characters for a variety of circumstances (my thanks to the CERT, who developed this list):

  • In the content of a block-level element (e.g., in the middle of a paragraph of text in HTML or a block in XML):

    • "<" is special because it introduces a tag.

    • "&" is special because it introduces a character entity.

    • ">" is special because some browsers treat it as special, on the assumption that the author of the page really meant to put in an opening "<", but omitted it in error.

  • In attribute values:

    • In attribute values enclosed with double quotes, the double quotes are special because they mark the end of the attribute value.

    • In attribute values enclosed with single quote, the single quotes are special because they mark the end of the attribute value. XML's definition allows single quotes, but I've been told that some XML parsers don't handle them correctly, so you might avoid using single quotes in XML.

    • Attribute values without any quotes make the white-space characters such as space and tab special. Note that these aren't legal in XML either, and they make more characters special. Thus, I recommend against unquoted attributes if you're using dynamically generated values in them.

    • "&" is special when used in conjunction with some attributes because it introduces a character entity.

  • In URLs, for example, a search engine might provide a link within the results page that the user can click to re-run the search. This can be implemented by encoding the search query inside the URL. When this is done, it introduces additional special characters:

    • Space, tab, and new line are special because they mark the end of the URL.

    • "&" is special because it introduces a character entity or separates CGI parameters.

    • Non-ASCII characters (that is, everything above 128 in the ISO-8859-1 encoding) aren't allowed in URLs, so they are all special here.

    • The "%" must be filtered from input anywhere parameters encoded with HTTP escape sequences are decoded by server-side code. The percent must be filtered if input such as "%68%65%6C%6C%6F" becomes "hello" when it appears on the web page in question.

  • Within the body of a <SCRIPT> </SCRIPT> the semicolon, parenthesis, curly braces, and new line should be filtered in situations where text could be inserted directly into a preexisting script tag.

  • Server-side scripts that convert any exclamation characters (!) in input to double-quote characters (") on output might require additional filtering.

Note that, in general, the ampersand (&) is special in HTML and XML. Filtering

One approach to handling these special characters is simply eliminating them (usually during input or output).

If you're already validating your input for valid characters (and you generally should), this is easily done by simply omitting the special characters from the list of valid characters. Here's an example in Perl of a filter that only accepts legal characters, and since the filter doesn't accept any special characters other than the space, it's quite acceptable for use in areas such as a quoted attribute:
 # Accept only legal characters:
  $summary =~ tr/A-Za-z0-9\ \.\://dc;

However, if you really want to strip away only the smallest number of characters, then you could create a subroutine to remove just those characters:
 sub remove_special_chars {
   local($s) = @_;
   $s =~ s/[\<\>\"\'\%\;\(\)\&\+]//g;
   return $s;
  # Sample use:
  $data = &remove_special_chars($data); Encoding (Quoting)

An alternative to removing the special characters is to encode them so that they don't have any special meaning. This has several advantages over filtering the characters, in particular, it prevents data loss. If the data is "mangled" by the process from the user's point of view, at least when the data is encoded it's possible to reconstruct the data that was originally sent.

HTML, XML, and SGML all use the ampersand ("&") character as a way to introduce encodings in the running text; this encoding is often called ``HTML encoding.'' To encode these characters, simply transform the special characters in your circumstance. Usually this means '<' becomes '&lt;', '>' becomes '&gt;', '&' becomes '&amp;', and '"' becomes '&quot;'. As noted above, although in theory '>' doesn't need to be quoted, because some browsers act on it (and fill in a '<') it needs to be quoted. There's a minor complexity with the double-quote character, because '&quot;' only needs to be used inside attributes, and some extremely old browsers don't properly render it. If you can handle the additional complexity, you can try to encode '"' only when you need to, but it's easier to simply encode it and ask users to upgrade their browsers. Few users will use such ancient browsers, and the double-quote character encoding has been a standard for a long time.

Scripting languages may consider implementing specialized auto-quoting types, the interesting approach developed in the web application framework Quixote. Quixote includes a "template" feature which allows easy mixing of HTML text and Python code; text generated by a template is passed back to the web browser as an HTML document. As of version 0.6, Quixote has two kinds of text (instead of a single kind as most such languages). Anything which appears in a literal, quoted string is of type "htmltext," and it is assumed to be exactly as the programmer wanted it to be (this is reasoble, since the programmer wrote it). Anything which takes the form of an ordinary Python string, however, is automatically quoted as the template is executed. As a result, text from a database or other external source is automatically quoted, and cannot be used for a cross-site scripting attack. Thus, Quixote implements a safe default - programmers no longer need to worry about quoting every bit of text that passes through the application (bugs involving too much quoting are less likely to be a security problem, and will be obvious in testing). Quixote uses an open source software license, but because of its venue identification it is probably GPL-incompatible, and is used by organizations such as the Linux Weekly News.

This approach to HTML encoding isn't quite enough encoding in some circumstances. As discussed in Section 9.5, you need to specify the output character encoding (the ``charset''). If some of your data is encoded using a different character encoding than the output character encoding, then you'll need to do something so your output uses a consistent and correct encoding. Also, you've selected an output encoding other than ISO-8859-1, then you need to make sure that any alternative encodings for special characters (such as "<") can't slip through to the browser. This is a problem with several character encodings, including popular ones like UTF-7 and UTF-8; see Section 5.9 for more information on how to prevent ``alternative'' encodings of characters. One way to deal with incompatible character encodings is to first translate the characters internally to ISO 10646 (which has the same character values as Unicode), and then using either numeric character references or character entity references to represent them:

  • A numeric character reference looks like "&#D;", where D is a decimal number, or "&#xH;" or "&#XH;", where H is a hexadecimal number. The number given is the ISO 10646 character id (which has the same character values as Unicode). Thus &#1048; is the Cyrillic capital letter "I". The hexadecimal system isn't supported in the SGML standard (ISO 8879), so I'd suggest using the decimal system for output. Also, although SGML specification permits the trailing semicolon to be omitted in some circumstances, in practice many systems don't handle it - so always include the trailing semicolon.

  • A character entity reference does the same thing but uses mnemonic names instead of numbers. For example, "&lt;" represents the < sign. If you're generating HTML, see the HTML specification which lists all mnemonic names.

Either system (numeric or character entity) works; I suggest using character entity references for '<', '>', '&', and '"' because it makes your code (and output) easier for humans to understand. Other than that, it's not clear that one or the other system is uniformly better. If you expect humans to edit the output by hand later, use the character entity references where you can, otherwise I'd use the decimal numeric character references just because they're easier to program. This encoding scheme can be quite inefficient for some languages (especially Asian languages); if that is your primary content, you might choose to use a different character encoding (charset), filter on the critical characters (e.g., "<") and ensure that no alternative encodings for critical characters are allowed.

URIs have their own encoding scheme, commonly called ``URL encoding.'' In this system, characters not permitted in URLs are represented using a percent sign followed by its two-digit hexadecimal value. To handle all of ISO 10646 (Unicode), it's recommended to first translate the codes to UTF-8, and then encode it. See Section 5.11.4 for more about validating URIs.

7.16. Foil Semantic Attacks

A ``semantic attack'' is an attack in which the attacker uses the computing infrastructure/system in a way that fools the victim into thinking they are doing something, but are doing something different, yet the computing infrastructure/system is working exactly as it was designed to do. Semantic attacks often involve financial scams, where the attacker is trying to fool the victim into giving the attacker large sums of money (e.g., thinking they're investing in something). For example, the attacker may try to convince the user that they're looking at a trusted website, even if they aren't.

Semantic attacks are difficult to counter, because they're exploiting the correct operation of the computer. The way to deal with semantic attacks is to help give the human additional information, so that when ``odd'' things happen the human will have more information or a warning will be presented that something may not be what it appears to be.

One example is URIs that, while legitimate, may fool users into thinking they have a different meaning. For example, look at this URI:
If a user clicked on that URI, they might think that they're going to Bloomberg (who provide financial commodities news), but instead they're going to (and providing the username, which will conveniently ignore). If the website then imitated the site, a user might be convinced that they're seeing the real thing (and make investment decisions based on attacker-controlled information). This depends on URIs being used in an unusual way - clickable URIs can have usernames, but usually don't. One solution for this case is for the web browser to detect such unusual URIs and create a pop-up confirmation widget, saying ``You are about to log into as user; do you wish to proceed?'' If the widget allows the user to change these entries, it provides additional functionality to the user as well as providing protection against that attack.

Another example is homographs, particularly international homographs. Certain letters look similar to each other, and these can be exploited as well. For example, since 0 (zero) and O (the letter O) look similar to each other, users may not realize that WWW.BLOOMBERG.COM and WWW.BL00MBERG.COM are different web addresses. Other similar-looking letters include 1 (one) and l (lower-case L). If international characters are allowed, the situation is worse. For example, many Cyrillic letters look essentially the same as Roman letters, but the computer will treat them differently. Currently most systems don't allow international characters in host names, but for various good reasons it's widely agreed that support for them will be necessary in the future. One proposed solution has been to diplay letters from different code regions using different colors - that way, users get more information visually. If the users look at URI, they will hopefully notice the strange coloring. [Gabrilovich 2002] However, this does show the essence of a semantic attack - it's difficult to defend against, precisely because the computers are working correctly.

7.17. Be Careful with Data Types

Be careful with the data types used, in particular those used in interfaces. For example, ``signed'' and ``unsigned'' values are treated differently in many languages (such as C or C++).

Chapter 8. Carefully Call Out to Other Resources


Do not put your trust in princes, in mortal men, who cannot save.

 Psalms 146:3 (NIV)

Practically no program is truly self-contained; nearly all programs call out to other programs for resources, such as programs provided by the operating system, software libraries, and so on. Sometimes this calling out to other resources isn't obvious or involves a great deal of ``hidden'' infrastructure which must be depended on, e.g., the mechanisms to implement dynamic libraries. Clearly, you must be careful about what other resources your program trusts and you must make sure that the way you send requests to them.

8.1. Call Only Safe Library Routines

Sometimes there is a conflict between security and the development principles of abstraction (information hiding) and reuse. The problem is that some high-level library routines may or may not be implemented securely, and their specifications won't tell you. Even if a particular implementation is secure, it may not be possible to ensure that other versions of the routine will be safe, or that the same interface will be safe on other platforms.

In the end, if your application must be secure, you must sometimes re-implement your own versions of library routines. Basically, you have to re-implement routines if you can't be sure that the library routines will perform the necessary actions you require for security. Yes, in some cases the library's implementation should be fixed, but it's your users who will be hurt if you choose a library routine that is a security weakness. If can, try to use the high-level interfaces when you must re-implement something - that way, you can switch to the high-level interface on systems where its use is secure.

If you can, test to see if the routine is secure or not, and use it if it's secure - ideally you can perform this test as part of compilation or installation (e.g., as part of an ``autoconf'' script). For some conditions this kind of run-time testing is impractical, but for other conditions, this can eliminate many problems. If you don't want to bother to re-implement the library, at least test to make sure it's safe and halt installation if it isn't. That way, users will not accidentally install an insecure program and will know what the problem is.

8.2. Limit Call-outs to Valid Values

Ensure that any call out to another program only permits valid and expected values for every parameter. This is more difficult than it sounds, because many library calls or commands call lower-level routines in potentially surprising ways. For example, many system calls are implemented indirectly by calling the shell, which means that passing characters which are shell metacharacters can have dangerous effects. So, let's discuss metacharacters.

8.3. Handle Metacharacters

Many systems, such as the command line shell and SQL interpreters, have ``metacharacters'', that is, characters in their input that are not interpreted as data. Such characters might commands, or delimit data from commands or other data. If there's a language specification for that system's interface that you're using, then it certainly has metacharacters. If your program invokes those other systems and allows attackers to insert such metacharacters, the usual result is that an attacker can completely control your program.

One of the most pervasive metacharacter problems are those involving shell metacharacters. The standard Unix-like command shell (stored in /bin/sh) interprets a number of characters specially. If these characters are sent to the shell, then their special interpretation will be used unless escaped; this fact can be used to break programs. According to the WWW Security FAQ [Stein 1999, Q37], these metacharacters are:
& ; ` ' \ " | * ? ~ < > ^ ( ) [ ] { } $ \n \r

I should note that in many situations you'll also want to escape the tab and space characters, since they (and the newline) are the default parameter separators. The separator values can be changed by setting the IFS environment variable, but if you can't trust the source of this variable you should have thrown it out or reset it anyway as part of your environment variable processing.

Unfortunately, in real life this isn't a complete list. Here are some other characters that can be problematic:

  • '!' means ``not'' in an expression (as it does in C); if the return value of a program is tested, prepending ! could fool a script into thinking something had failed when it succeeded or vice versa. In some shells, the "!" also accesses the command history, which can cause real problems. In bash, this only occurs for interactive mode, but tcsh (a csh clone found in some Linux distributions) uses "!" even in scripts.

  • '#' is the comment character; all further text on the line is ignored.

  • '-' can be misinterpreted as leading an option (or, as - -, disabling all further options). Even if it's in the ``middle'' of a filename, if it's preceded by what the shell considers as whitespace you may have a problem.

  • ' ' (space), '\t' (tab), '\n' (newline), '\r' (return), '\v' (vertical space), '\f' (form feed), and other whitespace characters can have many dangerous effects. They can may turn a ``single'' filename into multiple arguments, for example, or turn a single parameter into multiple parameter when stored. Newline and return have a number of additional dangers, for example, they can be used to create ``spoofed'' log entries in some programs, or inserted just before a separate command that is then executed (if an underlying protocol uses newlines or returns as command separators).

  • Other control characters (in particular, NIL) may cause problems for some shell implementations.

  • Depending on your usage, it's even conceivable that ``.'' (the ``run in current shell'') and ``='' (for setting variables) might be worrisome characters. However, any example I've found so far where these are issues have other (much worse) security problems.

What makes the shell metacharacters particularly pervasive is that several important library calls, such as popen(3) and system(3), are implemented by calling the command shell, meaning that they will be affected by shell metacharacters too. Similarly, execlp(3) and execvp(3) may cause the shell to be called. Many guidelines suggest avoiding popen(3), system(3), execlp(3), and execvp(3) entirely and use execve(3) directly in C when trying to spawn a process [Galvin 1998b]. At the least, avoid using system(3) when you can use the execve(3); since system(3) uses the shell to expand characters, there is more opportunity for mischief in system(3). In a similar manner the Perl and shell backtick (`) also call a command shell; for more information on Perl see Section 10.2.

Since SQL also has metacharacters, a similar issue revolves around calls to SQL. When metacharacters are provided as input to trigger SQL metacharacters, it's often called "SQL injection". See SPI Dynamic's paper ``SQL Injection: Are your Web Applications Vulnerable?'' for further discussion on this. As discussed in Chapter 5, define a very limited pattern and only allow data matching that pattern to enter; if you limit your pattern to ^[0-9]$ or ^[0-9A-Za-z]*$ then you won't have a problem. If you must handle data that may include SQL metacharacters, a good approach is to convert it (as early as possible) to some other encoding before storage, e.g., HTML encoding (in which case you'll need to encode any ampersand characters too). Also, prepend and append a quote to all user input, even if the data is numeric; that way, insertions of white space and other kinds of data won't be as dangerous.

Forgetting one of these characters can be disastrous, for example, many programs omit backslash as a shell metacharacter [rfp 1999]. As discussed in the Chapter 5, a recommended approach by some is to immediately escape at least all of these characters when they are input. But again, by far and away the best approach is to identify which characters you wish to permit, and use a filter to only permit those characters.

A number of programs, especially those designed for human interaction, have ``escape'' codes that perform ``extra'' activities. One of the more common (and dangerous) escape codes is one that brings up a command line. Make sure that these ``escape'' commands can't be included (unless you're sure that the specific command is safe). For example, many line-oriented mail programs (such as mail or mailx) use tilde (~) as an escape character, which can then be used to send a number of commands. As a result, apparently-innocent commands such as ``mail admin < file-from-user'' can be used to execute arbitrary programs. Interactive programs such as vi, emacs, and ed have ``escape'' mechanisms that allow users to run arbitrary shell commands from their session. Always examine the documentation of programs you call to search for escape mechanisms. It's best if you call only programs intended for use by other programs; see Section 8.4.

The issue of avoiding escape codes even goes down to low-level hardware components and emulators of them. Most modems implement the so-called ``Hayes'' command set. Unless the command set is disabled, inducing a delay, the phrase ``+++'', and then another delay forces the modem to interpret any following text as commands to the modem instead. This can be used to implement denial-of-service attacks (by sending ``ATH0'', a hang-up command) or even forcing a user to connect to someone else (a sophisticated attacker could re-route a user's connection through a machine under the attacker's control). For the specific case of modems, this is easy to counter (e.g., add "ATS2-255" in the modem initialization string), but the general issue still holds: if you're controlling a lower-level component, or an emulation of one, make sure that you disable or otherwise handle any escape codes built into them.

Many ``terminal'' interfaces implement the escape codes of ancient, long-gone physical terminals like the VT100. These codes can be useful, for example, for bolding characters, changing font color, or moving to a particular location in a terminal interface. However, do not allow arbitrary untrusted data to be sent directly to a terminal screen, because some of those codes can cause serious problems. On some systems you can remap keys (e.g., so when a user presses "Enter" or a function key it sends the command you want them to run). On some you can even send codes to clear the screen, display a set of commands you'd like the victim to run, and then send that set ``back'', forcing the victim to run the commands of the attacker's choosing without even waiting for a keystroke. This is typically implemented using ``page-mode buffering''. This security problem is why emulated tty's (represented as device files, usually in /dev/) should only be writeable by their owners and never anyone else - they should never have ``other write'' permission set, and unless only the user is a member of the group (i.e., the ``user-private group'' scheme), the ``group write'' permission should not be set either for the terminal [Filipski 1986]. If you're displaying data to the user at a (simulated) terminal, you probably need to filter out all control characters (characters with values less than 32) from data sent back to the user unless they're identified by you as safe. Worse comes to worse, you can identify tab and newline (and maybe carriage return) as safe, removing all the rest. Characters with their high bits set (i.e., values greater than 127) are in some ways trickier to handle; some old systems implement them as if they weren't set, but simply filtering them inhibits much international use. In this case, you need to look at the specifics of your situation.

A related problem is that the NIL character (character 0) can have surprising effects. Most C and C++ functions assume that this character marks the end of a string, but string-handling routines in other languages (such as Perl and Ada95) can handle strings containing NIL. Since many libraries and kernel calls use the C convention, the result is that what is checked is not what is actually used [rfp 1999].

When calling another program or referring to a file always specify its full path (e.g, /usr/bin/sort). For program calls, this will eliminate possible errors in calling the ``wrong'' command, even if the PATH value is incorrectly set. For other file referents, this reduces problems from ``bad'' starting directories.

8.4. Call Only Interfaces Intended for Programmers

Call only application programming interfaces (APIs) that are intended for use by programs. Usually a program can invoke any other program, including those that are really designed for human interaction. However, it's usually unwise to invoke a program intended for human interaction in the same way a human would. The problem is that programs's human interfaces are intentionally rich in functionality and are often difficult to completely control. As discussed in Section 8.3, interactive programs often have ``escape'' codes, which might enable an attacker to perform undesirable functions. Also, interactive programs often try to intuit the ``most likely'' defaults; this may not be the default you were expecting, and an attacker may find a way to exploit this.

Examples of programs you shouldn't normally call directly include mail, mailx, ed, vi, and emacs. At the very least, don't call these without checking their input first.

Usually there are parameters to give you safer access to the program's functionality, or a different API or application that's intended for use by programs; use those instead. For example, instead of invoking a text editor to edit some text (such as ed, vi, or emacs), use sed where you can.

8.5. Check All System Call Returns

Every system call that can return an error condition must have that error condition checked. One reason is that nearly all system calls require limited system resources, and users can often affect resources in a variety of ways. Setuid/setgid programs can have limits set on them through calls such as setrlimit(3) and nice(2). External users of server programs and CGI scripts may be able to cause resource exhaustion simply by making a large number of simultaneous requests. If the error cannot be handled gracefully, then fail safe as discussed earlier.

8.6. Avoid Using vfork(2)

The portable way to create new processes in Unix-like systems is to use the fork(2) call. BSD introduced a variant called vfork(2) as an optimization technique. In vfork(2), unlike fork(2), the child borrows the parent's memory and thread of control until a call to execve(2V) or an exit occurs; the parent process is suspended while the child is using its resources. The rationale is that in old BSD systems, fork(2) would actually cause memory to be copied while vfork(2) would not. Linux never had this problem; because Linux used copy-on-write semantics internally, Linux only copies pages when they changed (actually, there are still some tables that have to be copied; in most circumstances their overhead is not significant). Nevertheless, since some programs depend on vfork(2), recently Linux implemented the BSD vfork(2) semantics (previously vfork(2) had been an alias for fork(2)).

There are a number of problems with vfork(2). From a portability point-of-view, the problem with vfork(2) is that it's actually fairly tricky for a process to not interfere with its parent, especially in high-level languages. The ``not interfering'' requirement applies to the actual machine code generated, and many compilers generate hidden temporaries and other code structures that cause unintended interference. The result: programs using vfork(2) can easily fail when the code changes or even when compiler versions change.

For secure programs it gets worse on Linux systems, because Linux (at least 2.2 versions through 2.2.17) is vulnerable to a race condition in vfork()'s implementation. If a privileged process uses a vfork(2)/execve(2) pair in Linux to execute user commands, there's a race condition while the child process is already running as the user's UID, but hasn`t entered execve(2) yet. The user may be able to send signals, including SIGSTOP, to this process. Due to the semantics of vfork(2), the privileged parent process would then be blocked as well. As a result, an unprivileged process could cause the privileged process to halt, resulting in a denial-of-service of the privileged process' service. FreeBSD and OpenBSD, at least, have code to specifically deal with this case, so to my knowledge they are not vulnerable to this problem. My thanks to Solar Designer, who noted and documented this problem in Linux on the ``security-audit'' mailing list on October 7, 2000.

The bottom line with vfork(2) is simple: don't use vfork(2) in your programs. This shouldn't be difficult; the primary use of vfork(2) is to support old programs that needed vfork's semantics.

8.7. Counter Web Bugs When Retrieving Embedded Content

Some data formats can embed references to content that is automatically retrieved when the data is viewed (not waiting for a user to select it). If it's possible to cause this data to be retrieved through the Internet (e.g., through the World Wide Wide), then there is a potential to use this capability to obtain information about readers without the readers' knowledge, and in some cases to force the reader to perform activities without the reader's consent. This privacy concern is sometimes called a ``web bug.''

In a web bug, a reference is intentionally inserted into a document and used by the content author to track who, where, and how often a document is read. The author can also essentially watch how a ``bugged'' document is passed from one person to another or from one organization to another.

The HTML format has had this issue for some time. According to the Privacy Foundation:

Web bugs are used extensively today by Internet advertising companies on Web pages and in HTML-based email messages for tracking. They are typically 1-by-1 pixel in size to make them invisible on the screen to disguise the fact that they are used for tracking. However, they could be any image (using the img tag); other HTML tags that can implement web bugs, e.g., frames, form invocations, and scripts. By itself, invoking the web bug will provide the ``bugging'' site the reader IP address, the page that the reader visited, and various information about the browser; by also using cookies it's often possible to determine the specific identify of the reader. A survey about web bugs is available at

What is more concerning is that other document formats seem to have such a capability, too. When viewing HTML from a web site with a web browser, there are other ways of getting information on who is browsing the data, but when viewing a document in another format from an email few users expect that the mere act of reading the document can be monitored. However, for many formats, reading a document can be monitored. For example, it has been recently determined that Microsoft Word can support web bugs; see the Privacy Foundation advisory for more information . As noted in their advisory, recent versions of Microsoft Excel and Microsoft Power Point can also be bugged. In some cases, cookies can be used to obtain even more information.

Web bugs are primarily an issue with the design of the file format. If your users value their privacy, you probably will want to limit the automatic downloading of included files. One exception might be when the file itself is being downloaded (say, via a web browser); downloading other files from the same location at the same time is much less likely to concern users.

8.8. Hide Sensitive Information

Sensitive information should be hidden from prying eyes, both while being input and output, and when stored in the system. Sensitive information certainly includes credit card numbers, account balances, and home addresses, and in many applications also includes names, email addressees, and other private information.

Web-based applications should encrypt all communication with a user that includes sensitive information; the usual way is to use the "https:" protocol (HTTP on top of SSL or TLS). According to the HTTP 1.1 specification (IETF RFC 2616 section 15.1.3), authors of services which use the HTTP protocol should not use GET based forms for the submission of sensitive data, because this will cause this data to be encoded in the Request-URI. Many existing servers, proxies, and user agents will log the request URI in some place where it might be visible to third parties. Instead, use POST-based submissions, which are intended for this purpose.

Databases of such sensitive data should also be encrypted on any storage device (such as files on a disk). Such encryption doesn't protect against an attacker breaking the secure application, of course, since obviously the application has to have a way to access the encrypted data too. However, it does provide some defense against attackers who manage to get backup disks of the data but not of the keys used to decrypt them. It also provides some defense if an attacker doesn't manage to break into an application, but does manage to partially break into a related system just enough to view the stored data - again, they now have to break the encryption algorithm to get the data. There are many circumstances where data can be transferred unintentionally (e.g., core files), which this also prevents. It's worth noting, however, that this is not as strong a defense as you'd think, because often the server itself can be subverted or broken.

Chapter 9. Send Information Back Judiciously


Do not answer a fool according to his folly, or you will be like him yourself.

 Proverbs 26:4 (NIV)

9.1. Minimize Feedback

Avoid giving much information to untrusted users; simply succeed or fail, and if it fails just say it failed and minimize information on why it failed. Save the detailed information for audit trail logs. For example:

  • If your program requires some sort of user authentication (e.g., you're writing a network service or login program), give the user as little information as possible before they authenticate. In particular, avoid giving away the version number of your program before authentication. Otherwise, if a particular version of your program is found to have a vulnerability, then users who don't upgrade from that version advertise to attackers that they are vulnerable.

  • If your program accepts a password, don't echo it back; this creates another way passwords can be seen.

9.2. Don't Include Comments

When returning information, don't include any ``comments'' unless you're sure you want the receiving user to be able to view them. This is a particular problem for web applications that generate files (such as HTML). Often web application programmers wish to comment their work (which is fine), but instead of simply leaving the comment in their code, the comment is included as part of the generated file (usually HTML or XML) that is returned to the user. The trouble is that these comments sometimes provide insight into how the system works in a way that aids attackers.

9.3. Handle Full/Unresponsive Output

It may be possible for a user to clog or make unresponsive a secure program's output channel back to that user. For example, a web browser could be intentionally halted or have its TCP/IP channel response slowed. The secure program should handle such cases, in particular it should release locks quickly (preferably before replying) so that this will not create an opportunity for a Denial-of-Service attack. Always place time-outs on outgoing network-oriented write requests.

9.4. Control Data Formatting (Format Strings/Formatation)

A number of output routines in computer languages have a parameter that controls the generated format. In C, the most obvious example is the printf() family of routines (including printf(), sprintf(), snprintf(), fprintf(), and so on). Other examples in C include syslog() (which writes system log information) and setproctitle() (which sets the string used to display process identifier information). Many functions with names beginning with ``err'' or ``warn'', containing ``log'' , or ending in ``printf'' are worth considering. Python includes the "%" operation, which on strings controls formatting in a similar manner. Many programs and libraries define formatting functions, often by calling built-in routines and doing additional processing (e.g., glib's g_snprintf() routine).

Format languages are essentially little programming languages - so developers who let attackers control the format string are essentially running programs written by attackers! Surprisingly, many people seem to forget the power of these formatting capabilities, and use data from untrusted users as the formatting parameter. The guideline here is clear - never use unfiltered data from an untrusted user as the format parameter. Failing to follow this guideline usually results in a format string vulnerability (also called a formatation vulnerability). Perhaps this is best shown by example:
  /* Wrong way: */
   /* Right ways: */
   printf("%s", string_from_untrusted_user); /* safe */
   fputs(string_from_untrusted_user); /* better for simple strings */

If an attacker controls the formatting information, an attacker can cause all sorts of mischief by carefully selecting the format. The case of C's printf() is a good example - there are lots of ways to possibly exploit user-controlled format strings in printf(). These include buffer overruns by creating a long formatting string (this can result in the attacker having complete control over the program), conversion specifications that use unpassed parameters (causing unexpected data to be inserted), and creating formats which produce totally unanticipated result values (say by prepending or appending awkward data, causing problems in later use). A particularly nasty case is printf's %n conversion specification, which writes the number of characters written so far into the pointer argument; using this, an attacker can overwrite a value that was intended for printing! An attacker can even overwrite almost arbitrary locations, since the attacker can specify a ``parameter'' that wasn't actually passed. The %n conversion specification has been standard part of C since its beginning, is required by all C standards, and is used by real programs. In 2000, Greg KH did a quick search of source code and identified the programs BitchX (an irc client), Nedit (a program editor), and SourceNavigator (a program editor / IDE / Debugger) as using %n, and there are doubtless many more. Deprecating %n would probably be a good idea, but even without %n there can be significant problems. Many papers discuss these attacks in more detail, for example, you can see Avoiding security holes when developing an application - Part 4: format strings.

Since in many cases the results are sent back to the user, this attack can also be used to expose internal information about the stack. This information can then be used to circumvent stack protection systems such as StackGuard and ProPolice; StackGuard uses constant ``canary'' values to detect attacks, but if the stack's contents can be displayed, the current value of the canary will be exposed, suddenly making the software vulnerable again to stack smashing attacks.

A formatting string should almost always be a constant string, possibly involving a function call to implement a lookup for internationalization (e.g., via gettext's _()). Note that this lookup must be limited to values that the program controls, i.e., the user must be allowed to only select from the message files controlled by the program. It's possible to filter user data before using it (e.g., by designing a filter listing legal characters for the format string such as [A-Za-z0-9]), but it's usually better to simply prevent the problem by using a constant format string or fputs() instead. Note that although I've listed this as an ``output'' problem, this can cause problems internally to a program before output (since the output routines may be saving to a file, or even just generating internal state such as via snprintf()).

The problem of input formatting causing security problems is not an idle possibility; see CERT Advisory CA-2000-13 for an example of an exploit using this weakness. For more information on how these problems can be exploited, see Pascal Bouchareine's email article titled ``[Paper] Format bugs'', published in the July 18, 2000 edition of Bugtraq. As of December 2000, developmental versions of the gcc compiler support warning messages for insecure format string usages, in an attempt to help developers avoid these problems.

Of course, this all begs the question as to whether or not the internationalization lookup is, in fact, secure. If you're creating your own internationalization lookup routines, make sure that an untrusted user can only specify a legal locale and not something else like an arbitrary path.

Clearly, you want to limit the strings created through internationalization to ones you can trust. Otherwise, an attacker could use this ability to exploit the weaknesses in format strings, particularly in C/C++ programs. This has been an item of discussion in Bugtraq (e.g., see John Levon's Bugtraq post on July 26, 2000). For more information, see the discussion on permitting users to only select legal language values in Section 5.8.3.

Although it's really a programming bug, it's worth mentioning that different countries notate numbers in different ways, in particular, both the period (.) and comma (,) are used to separate an integer from its fractional part. If you save or load data, you need to make sure that the active locale does not interfere with data handling. Otherwise, a French user may not be able to exchange data with an English user, because the data stored and retrieved will use different separators. I'm unaware of this being used as a security problem, but it's conceivable.

9.5. Control Character Encoding in Output

In general, a secure program must ensure that it synchronizes its clients to any assumptions made by the secure program. One issue often impacting web applications is that they forget to specify the character encoding of their output. This isn't a problem if all data is from trusted sources, but if some of the data is from untrusted sources, the untrusted source may sneak in data that uses a different encoding than the one expected by the secure program. This opens the door for a cross-site malicious content attack; see Section 5.10 for more information.

CERT's tech tip on malicious code mitigation explains the problem of unspecified character encoding fairly well, so I quote it here:

Many web pages leave the character encoding ("charset" parameter in HTTP) undefined. In earlier versions of HTML and HTTP, the character encoding was supposed to default to ISO-8859-1 if it wasn't defined. In fact, many browsers had a different default, so it was not possible to rely on the default being ISO-8859-1. HTML version 4 legitimizes this - if the character encoding isn't specified, any character encoding can be used.

If the web server doesn't specify which character encoding is in use, it can't tell which characters are special. Web pages with unspecified character encoding work most of the time because most character sets assign the same characters to byte values below 128. But which of the values above 128 are special? Some 16-bit character-encoding schemes have additional multi-byte representations for special characters such as "<". Some browsers recognize this alternative encoding and act on it. This is "correct" behavior, but it makes attacks using malicious scripts much harder to prevent. The server simply doesn't know which byte sequences represent the special characters.

For example, UTF-7 provides alternative encoding for "<" and ">", and several popular browsers recognize these as the start and end of a tag. This is not a bug in those browsers. If the character encoding really is UTF-7, then this is correct behavior. The problem is that it is possible to get into a situation in which the browser and the server disagree on the encoding.

Thankfully, though explaining the issue is tricky, its resolution in HTML is easy. In the HTML header, simply specify the charset, like this example from CERT:
 <META http-equiv="Content-Type"
 content="text/html; charset=ISO-8859-1">
 <P>This is a sample HTML page

From a technical standpoint, an even better approach is to set the character encoding as part of the HTTP protocol output, though some libraries make this more difficult. This is technically better because it doesn't force the client to examine the header to determine a character encoding that would enable it to read the META information in the header. Of course, in practice a browser that couldn't read the META information given above and use it correctly would not succeed in the marketplace, but that's a different issue. In any case, this just means that the server would need to send as part of the HTTP protocol, a ``charset'' with the desired value. Unfortunately, it's hard to heartily recommend this (technically better) approach, because some older HTTP/1.0 clients did not deal properly with an explicit charset parameter. Although the HTTP/1.1 specification requires clients to obey the parameter, it's suspicious enough that you probably ought to use it as an adjunct to forcing the use of the correct character encoding, and not your sole mechanism.

9.6. Prevent Include/Configuration File Access

When developing web based applications, do not allow users to access (read) files such as the program include and configuration files. This data may provide enough information (e.g., passwords) to break into the system. Note that this guideline sometimes also applies to other kinds of applications. There are several actions you can take to do this, including:

  • Place the include/configuration files outside of the web documentation root (so that the web server will never serve the files). Really, this is the best approach unless there's some reason the files have to be inside the document root.

  • Configure the web server so it will not serve include files as text. For example, if you're using Apache, you can add a handler or an action for .inc files like so:
     <Files *.inc>
        Order allow,deny
        Deny from all

  • Place the include files in a protected directory (using .htaccess), and designate them as files that won't be served.

  • Use a filter to deny access to the files. For Apache, this can be done using:
     <Files ~ "\.phpincludes">
         Order allow,deny
         Deny from all
    If you need full regular expressions to match filenames, in Apache you could use the FilesMatch directive.

  • If your include file is a valid script file, which your server will parse, make sure that it doesn't act on user-supplied parameters and that it's designed to be secure.

These approaches won't protect you from users who have access to the directories your files are in if they are world-readable. You could change the permissions of the files so that only the uid/gid of the webserver can read these files. However, this approach won't work if the user can get the web server to run his own scripts (the user can just write scripts to access your files). Fundamentally, if your site is being hosted on a server shared with untrusted people, it's harder to secure the system. One approach is to run multiple web serving programs, each with different permissions; this provides more security but is painful in practice. Another approach is to set these files to be read only by your uid/gid, and have the server run scripts at ``your'' permission. This latter approach has its own problems: it means that certain parts of the server must have root privileges, and that the script may have more permissions than necessary.

Chapter 10. Language-Specific Issues


Undoubtedly there are all sorts of languages in the world, yet none of them is without meaning.

 1 Corinthians 14:10 (NIV)

There are many language-specific security issues. Many of them can be summarized as follows:

  • Turn on all relevant warnings and protection mechanisms available to you where practical. For compiled languages, this includes both compile-time mechanisms and run-time mechanisms. In general, security-relevant programs should compile cleanly with all warnings turned on.

  • If you can use a ``safe mode'' (e.g., a mode that limits the activities of the executable), do so. Many interpreted languages include such a mode. In general, don't depend on the safe mode to provide absolute protection; most language's safe modes have not been sufficiently analyzed for their security, and when they are, people usually discover many ways to exploit it. However, by writing your code so that it's secure out of safe mode, and then adding the safe mode, you end up with defense-in-depth (since in many cases, an attacker has to break both your application code and the safe mode).

  • Avoid dangerous and deprecated operations in the language. By ``dangerous'', I mean operations which are difficult to use correctly. For example, many languages include some mechanisms or functions that are ``magical'', that is, they try to infer the ``right'' thing to do using a heuristic - generally you should avoid them, because an attacker may be able to exploit the heuristic and do something dangerous instead of what was intended. A common error is an ``off-by-one'' error, in which the bound is off by one, and sometimes these result in exploitable errors. In general, write code in a way that minimizes the likelihood of off-by-one errors. If there are standard conventions in the language (e.g., for writing loops), use them.

  • Ensure that the languages' infrastructure (e.g., run-time library) is available and secured.

  • Languages that automatically garbage-collect strings should be especially careful to immediately erase secret data (in particular secret keys and passwords).

  • Know precisely the semantics of the operations that you are using. Look up each operation's semantics in its documentation. Do not ignore return values unless you're sure they cannot be relevant. Don't ignore the difference between ``signed'' and ``unsigned'' values. This is particularly difficult in languages which don't support exceptions, like C, but that's the way it goes.

10.1. C/C++

It is possible to develop secure code using C or C++, but both languages include fundamental design decisions that make it more difficult to write secure code. C and C++ easily permit buffer overflows, force programmers to do their own memory management, and are fairly lax in their typing systems. For systems programs (such as an operating system kernel), C and C++ are fine choices. For applications, C and C++ are often over-used. Strongly consider using an even higher-level language, at least for the majority of the application. But clearly, there are many existing programs in C and C++ which won't get completely rewritten, and many developers may choose to develop in C and C++.

One of the biggest security problems with C and C++ programs is buffer overflow; see Chapter 6 for more information. C has the additional weakness of not supporting exceptions, which makes it easy to write programs that ignore critical error situations.

Another problem with C and C++ is that developers have to do their own memory management (e.g., using malloc(), alloc(), free(), new, and delete), and failing to do it correctly may result in a security flaw. The more serious problem is that programs may erroneously free memory that should not be freed (e.g., because it's already been freed). This can result in an immediate crash or be exploitable, allowing an attacker to cause arbitrary code to be executed; see [Anonymous Phrack 2001]. Some systems (such as many GNU/Linux systems) don't protect against double-freeing at all by default, and it is not clear that those systems which attempt to protect themselves are truly unsubvertable. Although I haven't seen anything written on the subject, I suspect that using the incorrect call in C++ (e.g., mixing new and malloc()) could have similar effects. For example, on March 11, 2002, it was announced that the zlib library had this problem, affecting the many programs that use it. Thus, when testing programs on GNU/Linux, you should set the environment variable MALLOC_CHECK_ to 1 or 2, and you might consider executing your program with that environment variable set with 0, 1, 2. The reason for this variable is explained in GNU/Linux malloc(3) man page:

Recent versions of Linux libc (later than 5.4.23) and GNU libc (2.x) include a malloc implementation which is tunable via environment variables. When MALLOC_CHECK_ is set, a special (less efficient) implementation is used which is designed to be tolerant against simple errors, such as double calls of free() with the same argument, or overruns of a single byte (off-by-one bugs). Not all such errors can be protected against, however, and memory leaks can result. If MALLOC_CHECK_ is set to 0, any detected heap corruption is silently ignored; if set to 1, a diagnostic is printed on stderr; if set to 2, abort() is called immediately. This can be useful because otherwise a crash may happen much later, and the true cause for the problem is then very hard to track down.

There are various tools to deal with this, such as Electric Fence and Valgrind; see Section 11.7 for more information. If unused memory is not free'd, (e.g., using free()), that unused memory may accumulate - and if enough unused memory can accumulate, the program may stop working. As a result, the unused memory may be exploitable by attackers to create a denial of service. It's theoretically possible for attackers to cause memory to be fragmented and cause a denial of service, but usually this is a fairly impractical and low-risk attack.

Be as strict as you reasonably can when you declare types. Where you can, use ``enum'' to define enumerated values (and not just a ``char'' or ``int'' with special values). This is particularly useful for values in switch statements, where the compiler can be used to determine if all legal values have been covered. Where it's appropriate, use ``unsigned'' types if the value can't be negative.

One complication in C and C++ is that the character type ``char'' can be signed or unsigned (depending on the compiler and machine). When a signed char with its high bit set is saved in an integer, the result will be a negative number; in some cases this can be exploitable. In general, use ``unsigned char'' instead of char or signed char for buffers, pointers, and casts when dealing with character data that may have values greater than 127 (0x7f).

C and C++ are by definition rather lax in their type-checking support, but you can at least increase their level of checking so that some mistakes can be detected automatically. Turn on as many compiler warnings as you can and change the code to cleanly compile with them, and strictly use ANSI prototypes in separate header (.h) files to ensure that all function calls use the correct types. For C or C++ compilations using gcc, use at least the following as compilation flags (which turn on a host of warning messages) and try to eliminate all warnings (note that -O2 is used since some warnings can only be detected by the data flow analysis performed at higher optimization levels):
gcc -Wall -Wpointer-arith -Wstrict-prototypes -O2
You might want ``-W -pedantic'' too.

Many C/C++ compilers can detect inaccurate format strings. For example, gcc can warn about inaccurate format strings for functions you create if you use its __attribute__() facility (a C extension) to mark such functions, and you can use that facility without making your code non-portable. Here is an example of what you'd put in your header (.h) file:
 /* in header.h */
  #ifndef __GNUC__
  #  define __attribute__(x) /*nothing*/
  extern void logprintf(const char *format, ...)
  extern void logprintva(const char *format, va_list args)
The "format" attribute takes either "printf" or "scanf", and the numbers that follow are the parameter number of the format string and the first variadic parameter (respectively). The GNU docs talk about this well. Note that there are other __attribute__ facilities as well, such as "noreturn" and "const".

Avoid common errors made by C/C++ developers. For example, be careful about not using ``='' when you mean ``==''.

10.2. Perl

Perl programmers should first read the man page perlsec(1), which describes a number of issues involved with writing secure programs in Perl. In particular, perlsec(1) describes the ``taint'' mode, which most secure Perl programs should use. Taint mode is automatically enabled if the real and effective user or group IDs differ, or you can use the -T command line flag (use the latter if you're running on behalf of someone else, e.g., a CGI script). Taint mode turns on various checks, such as checking path directories to make sure they aren't writable by others.

The most obvious affect of taint mode, however, is that you may not use data derived from outside your program to affect something else outside your program by accident. In taint mode, all externally-obtained input is marked as ``tainted'', including command line arguments, environment variables, locale information (see perllocale(1)), results of certain system calls (readdir, readlink, the gecos field of getpw* calls), and all file input. Tainted data may not be used directly or indirectly in any command that invokes a sub-shell, nor in any command that modifies files, directories, or processes. There is one important exception: If you pass a list of arguments to either system or exec, the elements of that list are NOT checked for taintedness, so be especially careful with system or exec while in taint mode.

Any data value derived from tainted data becomes tainted also. There is one exception to this; the way to untaint data is to extract a substring of the tainted data. Don't just use ``.*'' blindly as your substring, though, since this would defeat the tainting mechanism's protections. Instead, identify patterns that identify the ``safe'' pattern allowed by your program, and use them to extract ``good'' values. After extracting the value, you may still need to check it (in particular for its length).

The open, glob, and backtick functions call the shell to expand filename wild card characters; this can be used to open security holes. You can try to avoid these functions entirely, or use them in a less-privileged ``sandbox'' as described in perlsec(1). In particular, backticks should be rewritten using the system() call (or even better, changed entirely to something safer).

The perl open() function comes with, frankly, ``way too much magic'' for most secure programs; it interprets text that, if not carefully filtered, can create lots of security problems. Before writing code to open or lock a file, consult the perlopentut(1) man page. In most cases, sysopen() provides a safer (though more convoluted) approach to opening a file. The new Perl 5.6 adds an open() call with 3 parameters to turn off the magic behavior without requiring the convolutions of sysopen().

Perl programs should turn on the warning flag (-w), which warns of potentially dangerous or obsolete statements.

You can also run Perl programs in a restricted environment. For more information see the ``Safe'' module in the standard Perl distribution. I'm uncertain of the amount of auditing that this has undergone, so beware of depending on this for security. You might also investigate the ``Penguin Model for Secure Distributed Internet Scripting'', though at the time of this writing the code and documentation seems to be unavailable.

Many installations include a setuid root version of perl named ``suidperl''. However, the perldelta man page version 5.6.1 recommends using sudo instead, stating the following:

"Note that suidperl is neither built nor installed by default in any recent version of perl. Use of suidperl is highly discouraged. If you think you need it, try alternatives such as sudo first. See".

10.3. Python

As with any language, beware of any functions which allow data to be executed as parts of a program, to make sure an untrusted user can't affect their input. This includes exec(), eval(), and execfile() (and frankly, you should check carefully any call to compile()). The input() statement is also surprisingly dangerous. [Watters 1996, 150].

Python programs with privileges that can be invoked by unprivileged users (e.g., setuid/setgid programs) must not import the ``user'' module. The user module causes the file to be read and executed. Since this file would be under the control of an untrusted user, importing the user module allows an attacker to force the trusted program to run arbitrary code.

Python does very little compile-time checking -- it has essentially no compile-time type information, and it doesn't even check that the number of parameters passed are legal for a given function or method. This is unfortunate, resulting in a lot of latent bugs (both John Viega and I have experienced this problem). Hopefully someday Python will implement optional static typing and type-checking, an idea that's been discussed for some time. A partial solution for now is PyChecker, a lint-like program that checks for common bugs in Python source code. You can get PyChecker from

Python includes support for ``Restricted Execution'' through its RExec class. This is primarily intended for executing applets and mobile code, but it can also be used to limit privilege in a program even when the code has not been provided externally. By default, a restricted execution environment permits reading (but not writing) of files, and does not include operations for network access or GUI interaction. These defaults can be changed, but beware of creating loopholes in the restricted environment. In particular, allowing a user to unrestrictedly add attributes to a class permits all sorts of ways to subvert the environment because Python's implementation calls many ``hidden'' methods. Note that, by default, most Python objects are passed by reference; if you insert a reference to a mutable value into a restricted program's environment, the restricted program can change the object in a way that's visible outside the restricted environment! Thus, if you want to give access to a mutable value, in many cases you should copy the mutable value or use the Bastion module (which supports restricted access to another object). For more information, see Kuchling [2000]. I'm uncertain of the amount of auditing that the restricted execution capability has undergone, so programmer beware.

10.4. Shell Scripting Languages (sh and csh Derivatives)

I strongly recommend against using standard command shell scripting languages (such as csh, sh, and bash) for setuid/setgid secure code. Some systems (such as Linux) completely disable setuid/setgid shell scripts, so creating setuid/setgid shell scripts creates an unnecessary portability problem. On some old systems they are fundamentally insecure due to a race condition (as discussed in Section 3.1.3). Even for other systems, they're not really a good idea.

In fact, there are a vast number of circumstances where shell scripting languages shouldn't be used at all for secure programs. Standard command shells are notorious for being affected by nonobvious inputs - generally because command shells were designed to try to do things ``automatically'' for an interactive user, not to defend against a determined attacker. Shell programs are fine for programs that don't need to be secure (e.g., they run at the same privilege as the unprivileged user and don't accept ``untrusted'' data). They can also be useful when they're running with privilege, as long as all the input (e.g., files, directories, command line, environment, etc.) are all from trusted users - which is why they're often used quite successfully in startup/shutdown scripts.

Writing secure shell programs in the presence of malicious input is harder than in many other languages because of all the things that shells are affected by. For example, ``hidden'' environment variables (e.g., the ENV, BASH_ENV, and IFS values) can affect how they operate or even execute arbitrary user-defined code before the script can even execute. Even things like filenames of the executable or directory contents can affect execution. If an attacker can create filenames containing some control characters (e.g., newline), or whitespace, or shell metacharacters, or begin with a dash (the option flag syntax), there are often ways to exploit them. For example, on many Bourne shell implementations, doing the following will grant root access (thanks to NCSA for describing this exploit):
 % ln -s /usr/bin/setuid-shell /tmp/-x
  % cd /tmp
  % -x
Some systems may have closed this hole, but the point still stands: most command shells aren't intended for writing secure setuid/setgid programs. For programming purposes, avoid creating setuid shell scripts, even on those systems that permit them. Instead, write a small program in another language to clean up the environment, then have it call other executables (some of which might be shell scripts).

If you still insist on using shell scripting languages, at least put the script in a directory where it cannot be moved or changed. Set PATH and IFS to known values very early in your script; indeed, the environment should be cleaned before the script is called. Also, very early on, ``cd'' to a safe directory. Use data only from directories that is controlled by trusted users, e.g., /etc, so that attackers can't insert maliciously-named files into those directories. Be sure to quote every filename passed on a command line, e.g., use "$1" not $1, because filenames with whitespace will be split. Call commands using "--" to disable additional options where you can, because attackers may create or pass filenames beginning with dash in the hope of tricking the program into processing it as an option. Be especially careful of filenames embedding other characters (e.g., newlines and other control characters). Examine input filenames especially carefully and be very restrictive on what filenames are permitted.

If you don't mind limiting your program to only work with GNU tools (or if you detect and optionally use the GNU tools instead when they are available), you might want to use NIL characters as the filename terminator instead of newlines. By using NIL characters, rather than whitespace or newlines, handling nasty filenames (e.g., those with embedded newlines) is much simpler. Several GNU tools that output or input filenames can use this format instead of the more common ``one filename per line'' format. Unfortunately, the name of this option isn't consistent between tools; for many tools the name of this option is ``--null'' or ``-0''. GNU programs xargs and cpio allow using either --null or -0, tar uses --null, find uses -print0, grep uses either --null or -Z, and sort uses either -z or --zero-terminated. Those who find this inconsistency particularly disturbing are invited to supply patches to the GNU authors; I would suggest making sure every program supported ``--null'' since that seems to be the most common option name. For example, here's one way to move files to a target directory, even if there may be a vast number of files and some may have awkward names with embedded newlines (thanks to Jim Dennis for reminding me of this):
 find . -print0 | xargs --null mv --target-dir=$TARG

In a similar vein, I recommend not trusting ``restricted shells'' to implement secure policies. Restricted shells are shells that intentionally prevent users from performing a large set of activities - their goal is to force users to only run a small set of programs. A restricted shell can be useful as a defense-in-depth measure, but restricted shells are notoriously hard to configure correctly and as configured are often subvertable. For example, some restricted shells will start by running some file in an unrestricted mode (e.g., ``.profile'') - if a user can change this file, they can force execution of that code. A restricted shell should be set up to only run a few programs, but if any of those programs have ``shell escapes'' to let users run more programs, attackers can use those shell escapes to escape the restricted shell. Even if the programs don't have shell escapes, it's quite likely that the various programs can be used together (along with the shell's capabilities) to escape the restrictions. Of course, if you don't set the PATH of a restricted shell (and allow any program to run), then an attacker can use the shell escapes of many programs (including text editors, mailers, etc.). The problem is that the purpose of a shell is to run other programs, but those other programs may allow unintended operations -- and the shell doesn't interpose itself to prevent these operations.

10.5. Ada

In Ada95, the Unbounded_String type is often more flexible than the String type because it is automatically resized as necessary. However, don't store especially sensitive secret values such as passwords or secret keys in an Unbounded_String, since core dumps and page areas might still hold them later. Instead, use the String type for this data, lock it into memory while it's used, and overwrite the data as soon as possible with some constant value such as (others => ' '). Use the Ada pragma Inspection_Point on the object holding the secret after erasing the memory. That way, you can be certain that the object containing the secret will really be erased (and that the the overwriting won't be optimized away).

It's common for beginning Ada programmers to believe that the String type's first index value is always 1, but this isn't true if the string is sliced. Avoid this error.

It's worth noting that SPARK is a ``high-integrity subset of the Ada programming language''; SPARK users use a tool called the ``SPARK Examiner'' to check conformance to SPARK rules, including flow analysis, and there are various supports for full formal proof of the code if desired. See the SPARK website for more information. To my knowledge, there are no OSS/FS SPARK tools. If you're storing passwords and private keys you should still lock them into memory if appropriate and overwrite them as soon as possible. Note that SPARK is often used in environments where paging does not occur.

10.6. Java

If you're developing secure programs using Java, frankly your first step (after learning Java) is to read the two primary texts for Java security, namely Gong [1999] and McGraw [1999] (for the latter, look particularly at section 7.1). You should also look at Sun's posted security code guidelines at, and there's a nice article by Sahu et al [2002] A set of slides describing Java's security model are freely available at You can also see McGraw [1998].

Obviously, a great deal depends on the kind of application you're developing. Java code intended for use on the client side has a completely different environment (and trust model) than code on a server side. The general principles apply, of course; for example, you must check and filter any input from an untrusted source. However, in Java there are some ``hidden'' inputs or potential inputs that you need to be wary of, as discussed below. Johnathan Nightingale [2000] made an interesting statement summarizing many of the issues in Java programming:

... the big thing with Java programming is minding your inheritances. If you inherit methods from parents, interfaces, or parents' interfaces, you risk opening doors to your code.

The following are a few key guidelines, based on Gong [1999], McGraw [1999], Sun's guidance, and my own experience:

  1. Do not use public fields or variables; declare them as private and provide accessors to them so you can limit their accessibility.

  2. Make methods private unless there is a good reason to do otherwise (and if you do otherwise, document why). These non-private methods must protect themselves, because they may receive tainted data (unless you've somehow arranged to protect them).

  3. The JVM may not actually enforce the accessibility modifiers (e.g., ``private'') at run-time in an application (as opposed to an applet). My thanks to John Steven (Cigital Inc.), who pointed this out on the ``Secure Programming'' mailing list on November 7, 2000. The issue is that it all depends on what class loader the class requesting the access was loaded with. If the class was loaded with a trusted class loader (including the null/ primordial class loader), the access check returns "TRUE" (allowing access). For example, this works (at least with Sun's 1.2.2 VM ; it might not work with other implementations):

    1. write a victim class (V) with a public field, compile it.

    2. write an 'attack' class (A) that accesses that field, compile it

    3. change V's public field to private, recompile

    4. run A - it'll access V's (now private) field.

    However, the situation is different with applets. If you convert A to an applet and run it as an applet (e.g., with appletviewer or browser), its class loader is no longer a trusted (or null) class loader. Thus, the code will throw java.lang.IllegalAccessError, with the message that you're trying to access a field V.secret from class A.

  4. Avoid using static field variables. Such variables are attached to the class (not class instances), and classes can be located by any other class. As a result, static field variables can be found by any other class, making them much more difficult to secure.

  5. Never return a mutable object to potentially malicious code (since the code may decide to change it). Note that arrays are mutable (even if the array contents aren't), so don't return a reference to an internal array with sensitive data.

  6. Never store user given mutable objects (including arrays of objects) directly. Otherwise, the user could hand the object to the secure code, let the secure code ``check'' the object, and change the data while the secure code was trying to use the data. Clone arrays before saving them internally, and be careful here (e.g., beware of user-written cloning routines).

  7. Don't depend on initialization. There are several ways to allocate uninitialized objects.

  8. Make everything final, unless there's a good reason not to. If a class or method is non-final, an attacker could try to extend it in a dangerous and unforeseen way. Note that this causes a loss of extensibility, in exchange for security.

  9. Don't depend on package scope for security. A few classes, such as java.lang, are closed by default, and some Java Virtual Machines (JVMs) let you close off other packages. Otherwise, Java classes are not closed. Thus, an attacker could introduce a new class inside your package, and use this new class to access the things you thought you were protecting.

  10. Don't use inner classes. When inner classes are translated into byte codes, the inner class is translated into a class accesible to any class in the package. Even worse, the enclosing class's private fields silently become non-private to permit access by the inner class!

  11. Minimize privileges. Where possible, don't require any special permissions at all. McGraw goes further and recommends not signing any code; I say go ahead and sign the code (so users can decide to ``run only signed code by this list of senders''), but try to write the program so that it needs nothing more than the sandbox set of privileges. If you must have more privileges, audit that code especially hard.

  12. If you must sign your code, put it all in one archive file. Here it's best to quote McGraw [1999]:

    The goal of this rule is to prevent an attacker from carrying out a mix-and-match attack in which the attacker constructs a new applet or library that links some of your signed classes together with malicious classes, or links together signed classes that you never meant to be used together. By signing a group of classes together, you make this attack more difficult. Existing code-signing systems do an inadequate job of preventing mix-and-match attacks, so this rule cannot prevent such attacks completely. But using a single archive can't hurt.

  13. Make your classes uncloneable. Java's object-cloning mechanism allows an attacker to instantiate a class without running any of its constructors. To make your class uncloneable, just define the following method in each of your classes:
    public final Object clone() throws java.lang.CloneNotSupportedException {
        throw new java.lang.CloneNotSupportedException();

    If you really need to make your class cloneable, then there are some protective measures you can take to prevent attackers from redefining your clone method. If you're defining your own clone method, just make it final. If you're not, you can at least prevent the clone method from being maliciously overridden by adding the following:
    public final void clone() throws java.lang.CloneNotSupportedException {

  14. Make your classes unserializeable. Serialization allows attackers to view the internal state of your objects, even private portions. To prevent this, add this method to your classes:
    private final void writeObject(ObjectOutputStream out)
       throws {
          throw new"Object cannot be serialized");

    Even in cases where serialization is okay, be sure to use the transient keyword for the fields that contain direct handles to system resources and that contain information relative to an address space. Otherwise, deserializing the class may permit improper access. You may also want to identify sensitive information as transient.

    If you define your own serializing method for a class, it should not pass an internal array to any DataInput/DataOuput method that takes an array. The rationale: All DataInput/DataOutput methods can be overridden. If a Serializable class passes a private array directly to a DataOutput(write(byte [] b)) method, then an attacker could subclass ObjectOutputStream and override the write(byte [] b) method to enable him to access and modify the private array. Note that the default serialization does not expose private byte array fields to DataInput/DataOutput byte array methods.

  15. Make your classes undeserializeable. Even if your class is not serializeable, it may still be deserializeable. An attacker can create a sequence of bytes that happens to deserialize to an instance of your class with values of the attacker's choosing. In other words, deserialization is a kind of public constructor, allowing an attacker to choose the object's state - clearly a dangerous operation! To prevent this, add this method to your classes:
    private final void readObject(ObjectInputStream in)
       throws {
         throw new"Class cannot be deserialized");

  16. Don't compare classes by name. After all, attackers can define classes with identical names, and if you're not careful you can cause confusion by granting these classes undesirable privileges. Thus, here's an example of the wrong way to determine if an object has a given class:
      if (obj.getClass().getName().equals("Foo")) {

    If you need to determine if two objects have exactly the same class, instead use getClass() on both sides and compare using the == operator, Thus, you should use this form:
      if (a.getClass() == b.getClass()) {
    If you truly need to determine if an object has a given classname, you need to be pedantic and be sure to use the current namespace (of the current class's ClassLoader). Thus, you'll need to use this format:
      if (obj.getClass() == this.getClassLoader().loadClass("Foo")) {

    This guideline is from McGraw and Felten, and it's a good guideline. I'll add that, where possible, it's often a good idea to avoid comparing class values anyway. It's often better to try to design class methods and interfaces so you don't need to do this at all. However, this isn't always practical, so it's important to know these tricks.

  17. Don't store secrets (cryptographic keys, passwords, or algorithm) in the code or data. Hostile JVMs can quickly view this data. Code obfuscation doesn't really hide the code from serious attackers.

10.7. Tcl

Tcl stands for ``tool command language'' and is pronounced ``tickle.'' Tcl is divided into two parts: a language and a library. The language is a simple language, originally intended for issuing commands to interactive programs and including basic programming capabilities. The library can be embedded in application programs. You can find more information about Tcl at sites such as the and the Tcl WWW Info web page and the comp.lang.tcl FAQ launch page at My thanks go to Wojciech Kocjan for providing some of this detailed information on using Tcl in secure applications.

For some security applications, especially interesting components of Tcl are Safe-Tcl (which creates a sandbox in Tcl) and Safe-TK (which implements a sandboxed portable GUI for Safe Tcl), as well as the WebWiseTclTk Toolkit which permits Tcl packages to be automatically located and loaded from anywhere on the World Wide Web. You can find more about the latter from It's not clear to me how much code review this has received.

Tcl's original design goal to be a small, simple language resulted in a language that was originally somewhat limiting and slow. For an example of the limiting weaknesses in the original language, see Richard Stallman's ``Why You Should Not Use Tcl''. For example, Tcl was originally designed to really support only one data type (string). Thankfully, these issues have been addressed over time. In particular, version 8.0 added support for more data types (integers are stored internally as integers, lists as lists and so on). This improves its capabilities, and in particular improves its speed.

As with essentially all scripting languages, Tcl has an "eval" command that parses and executes arbitrary Tcl commands. And like all such scripting languages, this eval command needs to be used especially carefully, or an attacker could insert characters in the input to cause malicious things to occur. For example, an attackers may be able insert characters with special meaning to Tcl such as embedded whitespace (including space and newline), double-quote, curly braces, square brackets, dollar signs, backslash, semicolon, or pound sign (or create input to cause these characters to be created during processing). This also applies to any function that passes data to eval as well (depending on how eval is called).

Here is a small example that may make this concept clearer; first, let's define a small function and then interactively invoke it directly - note that these uses are fine:
 proc something {a b c d e} {
        puts "A='$a'"
        puts "B='$b'"
        puts "C='$c'"
        puts "D='$d'"
        puts "E='$e'"
  % # This works normally:
  % something "test 1" "test2" "t3" "t4" "t5"
  A='test 1'
  % # Imagine that str1 is set by an attacker:
  % set str1 {test 1 [puts HELLOWORLD]}
  % # This works as well
  % something $str1 t2 t3 t4 t5
  A='test 1 [puts HELLOWORLD]'
However, continuing the example, let's see how "eval" can be incorrectly and correctly called. If you call eval in an incorrect (dangerous) way, it allows attackers to misuse it. However, by using commands like list or lrange to correctly group the input, you can avoid this problem:
 % # This is the WRONG way - str1 is interpreted.
  % eval something $str1 t2 t3
  % # Here's one solution, using "list".
  % eval something [list $str1 t2 t3 t4 t5]
  A='test 1 [puts HELLOWORLD]'
  % # Here's another solution, using lrange:
  % eval something [lrange $str1 0 end] t2
Using lrange is useful when concatenating arguments to a called function, e.g., with more complex libraries using callbacks. In Tcl, eval is often used to create a one-argument version of a function that takes a variable number of arguments, and you need to be careful when using it this way. Here's another example (presuming that you've defined a "printf" function):
 proc vprintf {str arglist} {
       eval printf [list $str] [lrange $arglist 0 end]
  % printf "1+1=%d  2+2=%d" 2 4
  % vprintf "1+1=%d  2+2=%d" {2 4}

Fundamentally, when passing a command that will be eventually evaluated, you must pass Tcl commands as a properly built list, and not as a (possibly concatentated) string. For example, the "after" command runs a Tcl command after a given number of milliseconds; if the data in $param1 can be controlled by an attacker, this Tcl code is dangerously wrong:
  # DON'T DO THIS if param1 can be controlled by an attacker
   after 1000 "someCommand someparam $param1"
This is wrong, because if an attacker can control the value of $param1, the attacker can control the program. For example, if the attacker can cause $param1 to have '[exit]', then the program will exit. Also, if $param1 would be '; exit', it would also exit.

Thus, the proper alternative would be:
 after 1000 [list someCommand someparam $param1]
Even better would be something like the following:
 set cmd [list someCommand someparam]
  after 1000 [concat $cmd $param1]

Here's another example showing what you shouldn't do, pretending that $params is data controlled by possibly malicious user:
 set params "%-20s TESTSTRING"
  puts "'[eval format $params]'"
will result in:
 'TESTSTRING       '
But, when if the untrusted user sends data with an embedded newline, like this:
 set params "%-20s TESTSTRING\nputs HELLOWORLD"
  puts "'[eval format $params]'"
The result will be this (notice that the attacker's code was executed!):
Wojciech Kocjan suggests that the simplest solution in this case is to convert this to a list using lrange, doing this:
 set params "%-20s TESTINGSTRING\nputs HELLOWORLD"
  puts "'[eval format [lrange $params 0 end]]'"
The result would be:
Note that this solution presumes that the potentially malicious text is concatenated to the end of the text; as with all languages, make sure the attacker cannot control the format text.

As a matter of style always use curly braces when using if, while, for, expr, and any other command which parses an argument using expr/eval/subst. Doing this will avoid a common error when using Tcl called unintended double substitution (aka double substitution). This is best explained by example; the following code is incorrect:
 while ![eof $file] {
      set line [gets $file]
The code is incorrect because the "![eof $file]" text will be evaluated by the Tcl parser when the while command is executed the first time, and not re-evaluated in every iteration as it should be. Instead, do this:
 while {![eof $file]} {
       set line [gets $file]
Note that both the condition, and the action to be performed, are surrounded by curly braces. Although there are cases where the braces are redundant, they never hurt, and when you fail to include the curly braces where they're needed (say, when making a minor change) subtle and hard-to-find errors often result.

More information on good Tcl style can be found in documents such as Ray Johnson's Tcl Style Guide.

In the past, I have stated that I don't recommend Tcl for writing programs which must mediate a security boundary. Tcl seems to have improved since that time, so while I cannot guarantee Tcl will work for your needs, I can't guarantee that any other language will work for you either. Again, my thanks to Wojciech Kocjan who provided some of these suggestions on how to write Tcl code for secure applications.

10.8. PHP

SecureReality has put out a very interesting paper titled ``A Study In Scarlet - Exploiting Common Vulnerabilities in PHP'' [Clowes 2001], which discusses some of the problems in writing secure programs in PHP, particularly in versions before PHP 4.1.0. Clowes concludes that ``it is very hard to write a secure PHP application (in the default configuration of PHP), even if you try''.

Granted, there are security issues in any language, but one particular issue stands out in older versions of PHP that arguably makes older PHP versions less secure than most languages: the way it loads data into its namespace. By default, in PHP (versions 4.1.0 and lower) all environment variables and values sent to PHP over the web are automatically loaded into the same namespace (global variables) that normal variables are loaded into - so attackers can set arbitrary variables to arbitrary values, which keep their values unless explicitly reset by a PHP program. In addition, PHP automatically creates variables with a default value when they're first requested, so it's common for PHP programs to not initialize variables. If you forget to set a variable, PHP can report it, but by default PHP won't - and note that this simply an error report, it won't stop an attacker who finds an unusual way to cause it. Thus, by default PHP allows an attacker to completely control the values of all variables in a program unless the program takes special care to override the attacker. Once the program takes over, it can reset these variables, but failing to reset any variable (even one not obvious) might open a vulnerability in the PHP program.

For example, the following PHP program (an example from Clowes) intends to only let those who know the password to get some important information, but an attacker can set ``auth'' in their web browser and subvert the authorization check:
   if ($pass == "hello")
    $auth = 1;
   if ($auth == 1)
    echo "some important information";

I and many others have complained about this particularly dangerous problem; it's particularly a problem because PHP is widely used. A language that's supposed to be easy to use better make it easy to write secure programs in, after all. It's possible to disable this misfeature in PHP by turning the setting ``register_globals'' to ``off'', but by default PHP versions up through 4.1.0 default set this to ``on'' and PHP before 4.1.0 is harder to use with register_globals off. The PHP developers warned in their PHP 4.1.0 announcenment that ``as of the next semi-major version of PHP, new installations of PHP will default to having register_globals set to off.'' This has now happened; as of PHP version 4.2.0, External variables (from the environment, the HTTP request, cookies or the web server) are no longer registered in the global scope by default. The preferred method of accessing these external variables is by using the new Superglobal arrays, introduced in PHP 4.1.0.

PHP with ``register_globals'' set to ``on'' is a dangerous choice for nontrivial programs - it's just too easy to write insecure programs. However, once ``register_globals'' is set to ``off'', PHP is quite a reasonable language for development.

The secure default should include setting ``register_globals'' to ``off'', and also including several functions to make it much easier for users to specify and limit the input they'll accept from external sources. Then web servers (such as Apache) could separately configure this secure PHP installation. Routines could be placed in the PHP library to make it easy for users to list the input variables they want to accept; some functions could check the patterns these variables must have and/or the type that the variable must be coerced to. In my opinion, PHP is a bad choice for secure web development if you set register_globals on.

As I suggested in earlier versions of this book, PHP has been trivially modified to become a reasonable choice for secure web development. However, note that PHP doesn't have a particularly good security vulnerability track record (e.g., register_globals, a file upload problem, and a format string problem in the error reporting library); I believe that security issues were not considered sufficiently in early editions of PHP; I also think that the PHP developers are now emphasizing security and that these security issues are finally getting worked out. One evidence is the major change that the PHP developers have made to get turn off register_globals; this had a significant impact on PHP users, and their willingness to make this change is a good sign. Unfortunately, it's not yet clear how secure PHP really is; PHP just hasn't had much of a track record now that the developers of PHP are examining it seriously for security issues. Hopefully this will become clear quickly.

If you've decided to use PHP, here are some of my recommendations (many of these recommendations are based on ways to counter the issues that Clowes raises):

  • Set the PHP configuration option ``register_globals'' off, and use PHP 4.2.0 or greater. PHP 4.1.0 adds several special arrays, particularly $_REQUEST, which makes it far simpler to develop software in PHP when ``register_globals'' is off. Setting register_globals off, which is the default in PHP 4.2.0, completely eliminates the most common PHP attacks. If you're assuming that register_globals is off, you should check for this first (and halt if it's not true) - that way, people who install your program will quickly know there's a problem. Note that many third-party PHP applications cannot work with this setting, so it can be difficult to keep it off for an entire website. It's possible to set register_globals off for only some programs. For example, for Apache, you could insert these lines into the file .htaccess in the PHP directory (or use Directory directives to control it further):
     php_flag register_globals Off
      php_flag track_vars On
    However, the .htaccess file itself is ignored unless the Apache web server is configured to permit overrides; often the Apache global configuration is set so that AllowOverride is set to None. So, for Apache users, if you can convince your web hosting service to set ``AllowOverride Options'' in their configuration file (often /etc/http/conf/http.conf) for your host, do that. Then write helper functions to simplify loading the data you need (and only that data).

  • If you must develop software where register_globals might be on while running (e.g., a widely-deployed PHP application), always set values not provided by the user. Don't depend on PHP default values, and don't trust any variable you haven't explicitly set. Note that you have to do this for every entry point (e.g., every PHP program or HTML file using PHP). The best approach is to begin each PHP program by setting all variables you'll be using, even if you're simply resetting them to the usual default values (like "" or 0). This includes global variables referenced in included files, even all libraries, transitively. Unfortunately, this makes this recommendation hard to do, because few developers truly know and understand all global variables that may be used by all functions they call. One lesser alternative is to search through HTTP_GET_VARS, HTTP_POST_VARS, HTTP_COOKIE_VARS, and HTTP_POST_FILES to see if the user provided the data - but programmers often forget to check all sources, and what happens if PHP adds a new data source (e.g., HTTP_POST_FILES wasn't in old versions of PHP). Of course, this simply tells you how to make the best of a bad situation; in case you haven't noticed yet, turn off register_globals!

  • Set the error reporting level to E_ALL, and resolve all errors reported by it during testing. Among other things, this will complain about un-initialized variables, which are a key issues in PHP. This is a good idea anyway whenever you start using PHP, because this helps debug programs, too. There are many ways to set the error reporting level, including in the ``php.ini'' file (global), the ``.htttpd.conf'' file (single-host), the ``.htaccess'' file (multi-host), or at the top of the script through the error_reporting function. I recommend setting the error reporting level in both the php.ini file and also at the top of the script; that way, you're protected if (1) you forget to insert the command at the top of the script, or (2) move the program to another machine and forget to change the php.ini file. Thus, every PHP program should begin like this:
      <?php error_reporting(E_ALL);?>
    It could be argued that this error reporting should be turned on during development, but turned off when actually run on a real site (since such error message could give useful information to an attacker). The problem is that if they're disabled during ``actual use'' it's all too easy to leave them disabled during development. So for the moment, I suggest the simple approach of simply including it in every entrance. A much better approach is to record all errors, but direct the error reports so they're only included in a log file (instead of having them reported to the attacker).

  • Filter any user information used to create filenames carefully, in particular to prevent remote file access. PHP by default comes with ``remote files'' functionality -- that means that file-opening commands like fopen(), that in other languages can only open local files, can actually be used to invoke web or ftp requests from another site.

  • Do not use old-style PHP file uploads; use the HTTP_POST_FILES array and related functions. PHP supports file uploads by uploading the file to some temporary directory with a special filename. PHP originally set a collection of variables to indicate where that filename was, but since an attacker can control variable names and their values, attackers could use that ability to cause great mischief. Instead, always use HTTP_POST_FILES and related functions to access uploaded files. Note that even in this case, PHP's approach permits attackers to temporarily upload files to you with arbitrary content, which is risky by itself.

  • Only place protected entry points in the document tree; place all other code (which should be most of it) outside the document tree. PHP has a history of unfortunate advice on this topic. Originally, PHP users were supposed to use the ``.inc'' (include) extension for ``included'' files, but these included files often had passwords and other information, and Apache would just give requesters the contents of the ``.inc'' files when asked to do so when they were in the document tree. Then developers gave all files a ``.php'' extension - which meant that the contents weren't seen, but now files never meant to be entry points became entry points and were sometimes exploitable. As mentioned earlier, the usual security advice is the best: place only the proected entry points (files) in the document tree, and place other code (e.g., libraries) outside the document tree. There shouldn't be any ``.inc'' files in the document tree at all.

  • Avoid the session mechanism. The ``session'' mechanism is handy for storing persistent data, but its current implementation has many problems. First, by default sessions store information in temporary files - so if you're on a multi-hosted system, you open yourself up to many attacks and revelations. Even those who aren't currently multi-hosted may find themselves multi-hosted later! You can "tie" this information into a database instead of the filesystem, but if others on a multi-hosted database can access that database with the same permissions, the problem is the same. There are also ambiguities if you're not careful (``is this the session value or an attacker's value''?) and this is another case where an attacker can force a file or key to reside on the server with content of their choosing - a dangerous situation - and the attacker can even control to some extent the name of the file or key where this data will be placed.

  • For all inputs, check that they match a pattern for acceptability (as with any language), and then use type casting to coerce non-string data into the type it should have. Develop ``helper'' functions to easily check and import a selected list of (expected) inputs. PHP is loosely typed, and this can cause trouble. For example, if an input datum has the value "000", it won't be equal to "0" nor is it empty(). This is particularly important for associative arrays, because their indexes are strings; this means that $data["000"] is different than $data["0"]. For example, to make sure $bar has type double (after making sure it only has the format legal for a double):
      $bar = (double) $bar; 

  • Be especially careful of risky functions. This includes those that perform PHP code execution (e.g., require(), include(), eval(), preg_replace()), command execution (e.g., exec(), passthru(), the backtick operator, system(), and popen()), and open files (e.g., fopen(), readfile(), and file()). This is not an exhaustive list!

  • Use magic_quotes_gpc() where appropriate - this eliminates many kinds of attacks.

  • Avoid file uploads, and consider modifying the php.ini file to disable them (file_uploads = Off). File uploads have had security holes in the past, so on older PHP's this is a necessity, and until more experience shows that they're safe this isn't a bad thing to remove. Remember, in general, to secure a system you should disable or remove anything you don't need.

Chapter 11. Special Topics


Understanding is a fountain of life to those who have it, but folly brings punishment to fools.

 Proverbs 16:22 (NIV)

11.1. Passwords

Where possible, don't write code to handle passwords. In particular, if the application is local, try to depend on the normal login authentication by a user. If the application is a CGI script, try to depend on the web server to provide the protection as much as possible - but see below about handling authentication in a web server. If the application is over a network, avoid sending the password as cleartext (where possible) since it can be easily captured by network sniffers and reused later. ``Encrypting'' a password using some key fixed in the algorithm or using some sort of shrouding algorithm is essentially the same as sending the password as cleartext.

For networks, consider at least using digest passwords. Digest passwords are passwords developed from hashes; typically the server will send the client some data (e.g., date, time, name of server), the client combines this data with the user password, the client hashes this value (termed the ``digest pasword'') and replies just the hashed result to the server; the server verifies this hash value. This works, because the password is never actually sent in any form; the password is just used to derive the hash value. Digest passwords aren't considered ``encryption'' in the usual sense and are usually accepted even in countries with laws constraining encryption for confidentiality. Digest passwords are vulnerable to active attack threats but protect against passive network sniffers. One weakness is that, for digest passwords to work, the server must have all the unhashed passwords, making the server a very tempting target for attack.

If your application permits users to set their passwords, check the passwords and permit only ``good'' passwords (e.g., not in a dictionary, having certain minimal length, etc.). You may want to look at information such as on how to choose a good password. You should use PAM if you can, because it supports pluggable password checkers.

11.2. Authenticating on the Web

On the web, a web server is usually authenticated to users by using SSL or TLS and a server certificate - but it's not as easy to authenticate who the users are. SSL and TLS do support client-side certificates, but there are many practical problems with actually using them (e.g., web browsers don't support a single user certificate format and users find it difficult to install them). You can learn about how to set up digital certificates from many places, e.g., Petbrain. Using Java or Javascript has its own problems, since many users disable them, some firewalls filter them out, and they tend to be slow. In most cases, requiring every user to install a plug-in is impractical too, though if the system is only for an intranet for a relatively small number of users this may be appropriate.

If you're building an intranet application, you should generally use whatever authentication system is used by your users. Unix-like systems tend to use Kerberos, NIS+, or LDAP. You may also need to deal with a Windows-based authentication schemes (which can be viewed as proprietary variants of Kerberos and LDAP). Thus, if your organization depend on Kerberos, design your system to use Kerberos. Try to separate the authentication system from the rest of your application, since the organization may (will!) change their authentication system over time.

Many techniques don't work or don't work very well. One approach that works in some cases is to use ``basic authentication'', which is built into essentially all browsers and servers. Unfortunately, basic authentication sends passwords unencrypted, so it makes passwords easy to steal; basic authentication by itself is really useful only for worthless information. You could store authentication information in the URLs selected by the users, but for most circumstances you should never do this - not only are the URLs sent unprotected over the wire (as with basic authentication), but there are too many other ways that this information can leak to others (e.g., through the browser history logs stored by many browsers, logs of proxies, and to other web sites through the Referer: field). You could wrap all communication with a web server using an SSL/TLS connection (which would encrypt it); this is secure (depending on how you do it), and it's necessary if you have important data, but note that this is costly in terms of performance. You could also use ``digest authentication'', which exposes the communication but at least authenticates the user without exposing the underlying password used to authenticate the user. Digest authentication is intended to be a simple partial solution for low-value communications, but digest authentication is not widely supported in an interoperable way by web browsers and servers. In fact, as noted in a March 18, 2002 eWeek article, Microsoft's web client (Internet Explorer) and web server (IIS) incorrectly implement the standard (RFC 2617), and thus won't work with other servers or browsers. Since Microsoft don't view this incorrect implementation as a serious problem, it will be a very long time before most of their customers have a correctly-working program.

Thus, the most common technique for authenticating on the web today is through cookies. Cookies weren't really designed for this purpose, but they can be used for authentication - but there are many wrong ways to use them that create security vulnerabilities, so be careful. For more information about cookies, see IETF RFC 2965, along with the older specifications about them. Note that to use cookies, some browsers (e.g., Microsoft Internet Explorer 6) may insist that you have a privacy profile (named p3p.xml on the root directory of the server).

Note that some users don't accept cookies, so this solution still has some problems. If you want to support these users, you should send this authentication information back and forth via HTML form hidden fields (since nearly all browsers support them without concern). You'd use the same approach as with cookies - you'd just use a different technology to have the data sent from the user to the server. Naturally, if you implement this approach, you need to include settings to ensure that these pages aren't cached for use by others. However, while I think avoiding cookies is preferable, in practice these other approaches often require much more development effort. Since it's so hard to implement this on a large scale for many application developers, I'm not currently stressing these approaches. I would rather describe an approach that is reasonably secure and reasonably easy to implement, than emphasize approaches that are too hard to implement correctly (by either developers or users). However, if you can do so without much effort, by all means support sending the authentication information using form hidden fields and an encrypted link (e.g., SSL/TLS). As with all cookies, for these cookies you should turn on the HttpOnly flag unless you have a web browser script that must be able to read the cookie.

Fu [2001] discusses client authentication on the web, along with a suggested approach, and this is the approach I suggest for most sites. The basic idea is that client authentication is split into two parts, a ``login procedure'' and ``subsequent requests.'' In the login procedure, the server asks for the user's username and password, the user provides them, and the server replies with an ``authentication token''. In the subsequent requests, the client (web browser) sends the authentication token to the server (along with its request); the server verifies that the token is valid, and if it is, services the request. Another good source of information about web authentication is Seifried [2001].

One serious problem with some web authentication techniques is that they are vulnerable to a problem called "session fixation". In a session fixation attack, the attacker fixes the user's session ID before the user even logs into the target server, thus eliminating the need to obtain the user's session ID afterwards. Basically, the attacker obtains an account, and then tricks another user into using the attacker's account - often by creating a special hypertext link and tricking the user into clicking on it. A good paper describing session fixation is the paper by Mitja Kolsek [2002]. A web authentication system you use should be resistant to session fixation.

11.2.1. Authenticating on the Web: Logging In

The login procedure is typically implemented as an HTML form; I suggest using the field names ``username'' and ``password'' so that web browsers can automatically perform some useful actions. Make sure that the password is sent over an encrypted connection (using SSL or TLS, through an https: connection) - otherwise, eavesdroppers could collect the password. Make sure all password text fields are marked as passwords in the HTML, so that the password text is not visible to anyone who can see the user's screen.

If both the username and password fields are filled in, do not try to automatically log in as that user. Instead, display the login form with the user and password fields; this lets the user verify that they really want to log in as that user. If you fail to do this, attackers will be able to exploit this weakness to perform a session fixation attack. Paranoid systems might want simply ignore the password field and make the user fill it in, but this interferes with browsers which can store passwords for users.

When the user sends username and password, it must be checked against the user account database. This database shouldn't store the passwords ``in the clear'', since if someone got a copy of the this database they'd suddenly get everyone's password (and users often reuse passwords). Some use crypt() to handle this, but crypt can only handle a small input, so I recommend using a different approach (this is my approach - Fu [2001] doesn't discuss this). Instead, the user database should store a username, salt, and the password hash for that user. The ``salt'' is just a random sequence of characters, used to make it harder for attackers to determine a password even if they get the password database - I suggest an 8-character random sequence. It doesn't need to be cryptographically random, just different from other users. The password hash should be computed by concatenating ``server key1'', the user's password, and the salt, and then running a cryptographically secure hash algorithm. Server key1 is a secret key unique to this server - keep it separate from the password database. Someone who has server key1 could then run programs to crack user passwords if they also had the password database; since it doesn't need to be memorized, it can be a long and complex password. Most secure would be HMAC-SHA-1 or HMAC-MD5; you could use SHA-1 (most web sites aren't really worried about the attacks it allows) or MD5 (but MD5 would be poorer choice; see the discussion about MD5).

Thus, when users create their accounts, the password is hashed and placed in the password database. When users try to log in, the purported password is hashed and compared against the hash in the database (they must be equal). When users change their password, they should type in both the old and new password, and the new password twice (to make sure they didn't mistype it); and again, make sure none of these password's characters are visible on the screen.

By default, don't save the passwords themselves on the client's web browser using cookies - users may sometimes use shared clients (say at some coffee shop). If you want, you can give users the option of ``saving the password'' on their browser, but if you do, make sure that the password is set to only be transmitted on ``secure'' connections, and make sure the user has to specifically request it (don't do this by default).

Make sure that the page is marked to not be cached, or a proxy server might re-serve that page to other users.

Once a user successfully logs in, the server needs to send the client an ``authentication token'' in a cookie, which is described next.

11.2.2. Authenticating on the Web: Subsequent Actions

Once a user logs in, the server sends back to the client a cookie with an authentication token that will be used from then on. A separate authentication token is used, so that users don't need to keep logging in, so that passwords aren't continually sent back and forth, and so that unencrypted communication can be used if desired. A suggested token (ignoring session fixation attacks) would look like this:
Where t is the expiration time of the token (say, in several hours), and data s identifies the user (say, the user name or session id). The digest is a keyed digest of the other fields. Feel free to change the field name of ``data'' to be more descriptive (e.g., username and/or sessionid). If you have more than one field of data (e.g., both a username and a sessionid), make sure the digest uses both the field names and data values of all fields you're authenticating; concatenate them with a pattern (say ``%%'', ``+'', or ``&'') that can't occur in any of the field data values. As described in a moment, it would be a good idea to include a username. The keyed digest should be a cryptographic hash of the other information in the token, keyed using a different server key2. The keyed digest should use HMAC-MD5 or HMAC-SHA1, using a different server key (key2), though simply using SHA1 might be okay for some purposes (or even MD5, if the risks are low). Key2 is subject to brute force guessing attacks, so it should be long (say 12+ characters) and unguessable; it does NOT need to be easily remembered. If this key2 is compromised, anyone can authenticate to the server, but it's easy to change key2 - when you do, it'll simply force currently ``logged in'' users to re-authenticate. See Fu [2001] for more details.

There is a potential weakness in this approach. I have concerns that Fu's approach, as originally described, is weak against session fixation attacks (from several different directions, which I don't want to get into here). Thus, I now suggest modifying Fu's approach and using this token format instead:
This is the same as the original Fu aproach, and older versions of this book (before December 2002) didn't suggest it. This modification adds a new "client" field to uniquely identify the client's current location/identity. The data in the client field should be something that should change if someone else tries to use the account; ideally, its new value should be unguessable, though that's hard to accomplish in practice. Ideally the client field would be the client's SSL client certificate, but currently that's a suggest that is hard to meet. At the least, it should be the user's IP address (as perceived from the server, and remember to plan for IPv6's longer addresses). This modification doesn't completely counter session fixation attacks, unfortunately (since if an attacker can determine what the user would send, the attacker may be able to make a request to a server and convince the client to accept those values). However, it does add resistance to the attack. Again, the digest must now include all the other data.

Here's an example. If a user logs into sucessfully, you might establish the expiration date as 2002-12-30T1800 (let's assume we'll transmit as ASCII text in this format for the moment), the username as "fred", the client session as "1234", and you might determine that the client's IP address was If you use a simple SHA-1 keyed digest (and use a key prefixing the rest of the data), with the server key2 value of "rM!V^m~v*Dzx", the digest could be computed over:
A keyed digest can be computed by running a cryptographic hash code over, say, the server key2, then the data; in this case, the digest would be:

From then on, the server must check the expiration time and recompute the digest of this authentication token, and only accept client requests if the digest is correct. If there's no token, the server should reply with the user login page (with a hidden form field to show where the successful login should go afterwards).

It would be prudent to display the username, especially on important screens, to help counter session fixation attacks. If users are given feedback on their username, they may notice if they don't have their expected username. This is helpful anyway if it's possible to have an unexpected username (e.g., a family that shares the same machine). Examples of important screens include those when a file is uploaded that should be kept private.

One odd implementation issue: although the specifications for the "Expires:" (expiration time) field for cookies permit time zones, it turns out that some versions of Microsoft's Internet Explorer don't implement time zones correctly for cookie expiration. Thus, you need to always use UTC time (also called Zulu time) in cookie expiration times for maximum portability. It's a good idea in general to use UTC time for time values, and convert when necessary for human display, since this eliminates other time zone and daylight savings time issues.

If you include a sessionid in the authentication token, you can limit access further. Your server could ``track'' what pages a user has seen in a given session, and only permit access to other appropriate pages from that point (e.g., only those directly linked from those page(s)). For example, if a user is granted access to page foo.html, and page foo.html has pointers to resources bar1.jpg and bar2.png, then accesses to bar4.cgi can be rejected. You could even kill the session, though only do this if the authentication information is valid (otherwise, this would make it possible for attackers to cause denial-of-service attacks on other users). This would somewhat limit the access an attacker has, even if they successfully hijack a session, though clearly an attacker with time and an authentication token could ``walk'' the links just as a normal user would.

One decision is whether or not to require the authentication token and/or data to be sent over a secure connection (e.g., SSL). If you send an authentication token in the clear (non-secure), someone who intercepts the token could do whatever the user could do until the expiration time. Also, when you send data over an unencrypted link, there's the risk of unnoticed change by an attacker; if you're worried that someone might change the data on the way, then you need to authenticate the data being transmitted. Encryption by itself doesn't guarantee authentication, but it does make corruption more likely to be detected, and typical libraries can support both encryption and authentication in a TLS/SSL connection. In general, if you're encrypting a message, you should also authenticate it. If your needs vary, one alternative is to create two authentication tokens - one is used only in a ``secure'' connection for important operations, while the other used for less-critical operations. Make sure the token used for ``secure'' connections is marked so that only secure connections (typically encrypted SSL/TLS connections) are used. If users aren't really different, the authentication token could omit the ``data'' entirely.

Again, make sure that the pages with this authentication token aren't cached. There are other reasonable schemes also; the goal of this text is to provide at least one secure solution. Many variations are possible.

11.2.3. Authenticating on the Web: Logging Out

You should always provide users with a mechanism to ``log out'' - this is especially helpful for customers using shared browsers (say at a library). Your ``logout'' routine's task is simple - just unset the client's authentication token.

11.3. Random Numbers

In many cases secure programs must generate ``random'' numbers that cannot be guessed by an adversary. Examples include session keys, public or private keys, symmetric keys, nonces and IVs used in many protocols, salts, and so on. Ideally, you should use a truly random source of data for random numbers, such as values based on radioactive decay (through precise timing of Geiger counter clicks), atmospheric noise, or thermal noise in electrical circuits. Some computers have a hardware component that functions as a real random value generator, and if it's available you should use it.

However, most computers don't have hardware that generates truly random values, so in most cases you need a way to generate random numbers that is sufficiently random that an adversary can't predict it. In general, this means that you'll need three things:

  • An ``unguessable'' state; typically this is done by measuring variances in timing of low-level devices (keystrokes, disk drive arm jitter, etc.) in a way that an adversary cannot control.

  • A cryptographically strong pseudo-random number generator (PRNG), which uses the state to generate ``random'' numbers.

  • A large number of bits (in both the seed and the resulting value used). There's no point in having a strong PRNG if you only have a few possible values, because this makes it easy for an attacker to use brute force attacks. The number of bits necessary varies depending on the circumstance, however, since these are often used as cryptographic keys, the normal rules of thumb for keys apply. For a symmetric key (result), I'd use at least 112 bits (3DES), 128 bits is a little better, and 160 bits or more is even safer.

Typically the PRNG uses the state to generate some values, and then some of its values and other unguessable inputs are used to update the state. There are lots of ways to attack these systems. For example, if an attacker can control or view inputs to the state (or parts of it), the attacker may be able to determine your supposedly ``random'' number.

A real danger with PRNGs is that most computer language libraries include a large set of pseudo-random number generators (PRNGs) which are inappropriate for security purposes. Let me say it again: do not use typical random number generators for security purposes. Typical library PRNGs are intended for use in simulations, games, and so on; they are not sufficiently random for use in security functions such as key generation. Most non-cryptographic library PRNGs are some variation of ``linear congruential generators'', where the ``next'' random value is computed as "(aX+b) mod m" (where X is the previous value). Good linear congruential generators are fast and have useful statistical properties, making them appropriate for their intended uses. The problem with such PRNGs is that future values can be easily deduced by an attacker (though they may appear random). Other algorithms for generating random numbers quickly, such as quadratic generators and cubic generators, have also been broken [Schneier 1996]. In short, you have to use cryptographically strong PRNGs to generate random numbers in secure applications - ordinary random number libraries are not sufficient.

Failing to correctly generate truly random values for keys has caused a number of problems, including holes in Kerberos, the X window system, and NFS [Venema 1996].

If possible, you should use system services (typically provided by the operating system) that are expressly designed to create cryptographically secure random values. For example, the Linux kernel (since 1.3.30) includes a random number generator, which is sufficient for many security purposes. This random number generator gathers environmental noise from device drivers and other sources into an entropy pool. When accessed as /dev/random, random bytes are only returned within the estimated number of bits of noise in the entropy pool (when the entropy pool is empty, the call blocks until additional environmental noise is gathered). When accessed as /dev/urandom, as many bytes as are requested are returned even when the entropy pool is exhausted. If you are using the random values for cryptographic purposes (e.g., to generate a key) on Linux, use /dev/random. *BSD systems also include /dev/random. Solaris users with the SUNWski package also have /dev/random. Note that if a hardware random number generator is available and its driver is installed, it will be used instead. More information is available in the system documentation random(4).

On other systems, you'll need to find another way to get truly random results. One possibility for other Unix-like systems is the Entropy Gathering Daemon (EGD), which monitors system activity and hashes it into random values; you can get it at You might consider using a cryptographic hash functions (e.g., SHA-1) on PRNG outputs. By using a hash algorithm, even if the PRNG turns out to be guessable, this means that the attacker must now also break the hash function.

If you have to implement a strong PRNG yourself, a good choice for a cryptographically strong (and patent-unencumbered) PRNG is the Yarrow algorithm; you can learn more about Yarrow from Some other PRNGs can be useful, but many widely-used ones have known weaknesses that may or may not matter depending on your application. Before implementing a PRNG yourself, consult the literature, such as [Kelsey 1998] and [McGraw 2000a]. You should also examine IETF RFC 1750. NIST has some useful information; see the NIST publication 800-22 and NIST errata. You should know about the diehard tests too. You might want to examine the paper titled "how Intel checked its PRNG", but unfortunately that paper appears to be unavailable now.

11.4. Specially Protect Secrets (Passwords and Keys) in User Memory

If your application must handle passwords or non-public keys (such as session keys, private keys, or secret keys), try to hide them and overwrite them immediately after using them so they have minimal exposure.

Systems such as Linux support the mlock() and mlockall() calls to keep memory from being paged to disk (since someone might acquire the kep later from the swap file). Note that on Linux this is a privileged system call, which causes its own issues (do I grant the program superuser privileges so it can call mlock, if it doesn't need them otherwise?).

Also, if your program handles such secret values, be sure to disable creating core dumps (via ulimit). Otherwise, an attacker may be able to halt the program and find the secret value in the data dump.

Beware - normally processes can monitor other processes through the calls for debuggers (e.g., via ptrace(2) and the /proc pseudo-filesystem) [Venema 1996] Kernels usually protect against these monitoring routines if the process is setuid or setgid (on the few ancient ones that don't, there really isn't a good way to defend yourself other than upgrading). Thus, if your process manages secret values, you probably should make it setgid or setuid (to a different unprivileged group or user) to forceably inhibit this kind of monitoring. Unless you need it to be setuid, use setgid (since this grants fewer privileges).

Then there's the problem of being able to actually overwrite the value, which often becomes language and compiler specific. In many languages, you need to make sure that you store such information in mutable locations, and then overwrite those locations. For example, in Java, don't use the type String to store a password because Strings are immutable (they will not be overwritten until garbage-collected and then reused, possibly a far time in the future). Instead, in Java use char[] to store a password, so it can be immediately overwritten. In Ada, use type String (an array of characters), and not type Unbounded_String, to make sure that you have control over the contents.

In many languages (including C and C++), be careful that the compiler doesn't optimize away the "dead code" for overwriting the value - since in this case it's not dead code. Many compilers, including many C/C++ compilers, remove writes to stores that are no longer used - this is often referred to as "dead store removal." Unfortunately, if the write is really to overwrite the value of a secret, this means that code that appears to be correct will be silently discareded. Ada provides the pragma Inspection_Point; place this after the code erasing the memory, and that way you can be certain that the object containing the secret will really be erased (and that the the overwriting won't be optimized away).

A Bugtraq post by Andy Polyakov (November 7, 2002) reported that the C/C++ compilers gcc version 3 or higher, SGI MIPSpro, and the Microsoft compilers eliminated simple inlined calls to memset intended to overwrite secrets. This is allowed by the C and C++ standards. Other C/C++ compilers (such as gcc less than version 3) preserved the inlined call to memset at all optimization levels, showing that the issue is compiler-specific. Simply declaring that the destination data is volatile doesn't help on all compilers; both the MIPSpro and Microsoft compilers ignored simple "volatilization". Simply "touching" the first byte of the secret data doesn't help either; he found that the MIPSpro and GCC>=3 cleverly nullify only the first byte and leave the rest intact (which is actually quite clever - the problem is that the compiler's cleverness is interfering with our goals). One approach that seems to work on all platforms is to write your own implementation of memset with internal "volatilization" of the first argument (this code is based on a workaround proposed by Michael Howard):
 void *guaranteed_memset(void *v,int c,size_t n)
   { volatile char *p=v; while (n--) *p++=c; return v; }
Then place this definition into an external file to force the function to be external (define the function in a corresponding .h file, and #include the file in the callers, as is usual). This approach appears to be safe at any optimization level (even if the function gets inlined).

11.5. Cryptographic Algorithms and Protocols

Often cryptographic algorithms and protocols are necessary to keep a system secure, particularly when communicating through an untrusted network such as the Internet. Where possible, use cryptographic techniques to authenticate information and keep the information private (but don't assume that simple encryption automatically authenticates as well). Generally you'll need to use a suite of available tools to secure your application.

For background information and code, you should probably look at the classic text ``Applied Cryptography'' [Schneier 1996]. The newsgroup ``sci.crypt'' has a series of FAQ's; you can find them at many locations, including Linux-specific resources include the Linux Encryption HOWTO at A discussion on how protocols use the basic algorithms can be found in [Opplinger 1998]. A useful collection of papers on how to apply cryptography in protocols can be found in [Stallings 1996]. What follows here is just a few comments; these areas are rather specialized and covered more thoroughly elsewhere.

Cryptographic protocols and algorithms are difficult to get right, so do not create your own. Instead, where you can, use protocols and algorithms that are widely-used, heavily analyzed, and accepted as secure. When you must create anything, give the approach wide public review and make sure that professional security analysts examine it for problems. In particular, do not create your own encryption algorithms unless you are an expert in cryptology, know what you're doing, and plan to spend years in professional review of the algorithm. Creating encryption algorithms (that are any good) is a task for experts only.

A number of algorithms are patented; even if the owners permit ``free use'' at the moment, without a signed contract they can always change their minds later, putting you at extreme risk later. In general, avoid all patented algorithms - in most cases there's an unpatented approach that is at least as good or better technically, and by doing so you avoid a large number of legal problems.

Another complication is that many counties regulate or restrict cryptography in some way. A survey of legal issues is available at the ``Crypto Law Survey'' site,

Often, your software should provide a way to reject ``too small'' keys, and let the user set what ``too small'' is. For RSA keys, 512 bits is too small for use. There is increasing evidence that 1024 bits for RSA keys is not enough either; Bernstein has suggested techniques that simplify brute-forcing RSA, and other work based on it (such as Shamir and Tromer's "Factoring Large Numbers with the TWIRL device") now suggests that 1024 bit keys can be broken in a year by a $10 Million device. You may want to make 2048 bits the minimum for RSA if you really want a secure system, and you should certainly do so if you plan to use those keys after 2015. For more about RSA specifically, see RSA's commentary on Bernstein's work. For a more general discussion of key length and other general cryptographic algorithm issues, see NIST's key management workshop in November 2001.

11.5.1. Cryptographic Protocols

When you need a security protocol, try to use standard-conforming protocols such as IPSec, SSL (soon to be TLS), SSH, S/MIME, OpenPGP/GnuPG/PGP, and Kerberos. Each has advantages and disadvantages; many of them overlap somewhat in functionality, but each tends to be used in different areas:

  • Internet Protocol Security (IPSec). IPSec provides encryption and/or authentication at the IP packet level. However, IPSec is often used in a way that only guarantees authenticity of two communicating hosts, not of the users. As a practical matter, IPSec usually requires low-level support from the operating system (which not all implement) and an additional keyring server that must be configured. Since IPSec can be used as a "tunnel" to secure packets belonging to multiple users and multiple hosts, it is especially useful for building a Virtual Private Network (VPN) and connecting a remote machine. As of this time, it is much less often used to secure communication from individual clients to servers. The new version of the Internet Protocol, IPv6, comes with IPSec ``built in,'' but IPSec also works with the more common IPv4 protocol. Note that if you use IPSec, don't use the encryption mode without the authentication, because the authentication also acts as integrity protection.

  • Secure Socket Layer (SSL) / TLS. SSL/TLS works over TCP and tunnels other protocols using TCP, adding encryption, authentication of the server, and optional authentication of the client (but authenticating clients using SSL/TLS requires that clients have configured X.509 client certificates, something rarely done). SSL version 3 is widely used; TLS is a later adjustment to SSL that strengthens its security and improves its flexibility. Currently there is a slow transition going on from SSLv3 to TLS, aided because implementations can easily try to use TLS and then back off to SSLv3 without user intervention. Unfortunately, a few bad SSLv3 implementations cause problems with the backoff, so you may need a preferences setting to allow users to skip using TLS if necessary. Don't use SSL version 2, it has some serious security weaknesses.

    SSL/TLS is the primary method for protecting http (web) transactions. Any time you use an "https://" URL, you're using SSL/TLS. Other protocols that often use SSL/TLS include POP3 and IMAP. SSL/TLS usually use a separate TCP/IP port number from the unsecured port, which the IETF is a little unhappy about (because it consumes twice as many ports; there are solutions to this). SSL is relatively easy to use in programs, because most library implementations allow programmers to use operations similar to the operations on standard sockets like SSL_connect(), SSL_write(), SSL_read(), etc. A widely used OSS/FS implementation of SSL (as well as other capabilities) is OpenSSL, available at

  • OpenPGP and S/MIME. There are two competing, essentially incompatible standards for securing email: OpenPGP and S/MIME. OpenPHP is based on the PGP application; an OSS/FS implementation is GNU Privacy Guard from Currently, their certificates are often not interchangeable; work is ongoing to repair this.

  • SSH. SSH is the primary method of securing ``remote terminals'' over an internet, and it also includes methods for tunelling X Windows sessions. However, it's been extended to support single sign-on and general secure tunelling for TCP streams, so it's often used for securing other data streams too (such as CVS accesses). The most popular implementation of SSH is OpenSSH, which is OSS/FS. Typical uses of SSH allows the client to authenticate that the server is truly the server, and then the user enters a password to authenticate the user (the password is encrypted and sent to the other system for verification). Current versions of SSH can store private keys, allowing users to not enter the password each time. To prevent man-in-the-middle attacks, SSH records keying information about servers it talks to; that means that typical use of SSH is vulnerable to a man-in-the-middle attack during the very first connection, but it can detect problems afterwards. In contrast, SSL generally uses a certificate authority, which eliminates the first connection problem but requires special setup (and payment!) to the certificate authority.

  • Kerberos. Kerberos is a protocol for single sign-on and authenticating users against a central authentication and key distribution server. Kerberos works by giving authenticated users "tickets", granting them access to various services on the network. When clients then contact servers, the servers can verify the tickets. Kerberos is a primary method for securing and supporting authentication on a LAN, and for establishing shared secrets (thus, it needs to be used with other algorithms for the actual protection of communication). Note that to use Kerberos, both the client and server have to include code to use it, and since not everyone has a Kerberos setup, this has to be optional - complicating the use of Kerberos in some programs. However, Kerberos is widely used.

Many of these protocols allow you to select a number of different algorithms, so you'll still need to pick reasonable defaults for algorithms (e.g., for encryption).

11.5.2. Symmetric Key Encryption Algorithms

The use, export, and/or import of implementations of encryption algorithms are restricted in many countries, and the laws can change quite rapidly. Find out what the rules are before trying to build applications using cryptography.

For secret key (bulk data) encryption algorithms, use only encryption algorithms that have been openly published and withstood years of attack, and check on their patent status. I would recommend using the new Advanced Encryption Standard (AES), also known as Rijndahl -- a number of cryptographers have analyzed it and not found any serious weakness in it, and I believe it has been through enough analysis to be trustworthy now. However, in August 2002 researchers Fuller and Millar discovered a mathematical property of the cipher that, while not an attack, might be exploitable into an attack (the approach may actually has serious consequences for some other algorithms, too). Thus, it's worth staying tuned to future work. A good alternative to AES is the Serpent algorithm, which is slightly slower but is very resistant to attack. For many applications triple-DES is a very good encryption algorithm; it has a reasonably lengthy key (112 bits), no patent issues, and a very long history of withstanding attacks (it's withstood attacks far longer than any other encryption algorithm with reasonable key length in the public literature, so it's probably the safest publicly-available symmetric encryption algorithm when properly implemented). However, triple-DES is very slow when implemented in software, so triple-DES can be considered ``safest but slowest.'' Twofish appears to be a good encryption algorithm, but there are some lingering questions - Sean Murphy and Fauzan Mirza showed that Twofish has properties that cause many academics to be concerned (though as of yet no one has managed to exploit these properties). MARS is highly resistent to ``new and novel'' attacks, but it's more complex and is impractical on small-ability smartcards. For the moment I would avoid Twofish - it's quite likely that this will never be exploitable, but it's hard to be sure and there are alternative algorithms which don't have these concerns. Don't use IDEA - it's subject to U.S. and European patents. Don't use stupid algorithms such as XOR with a constant or constant string, the ROT (rotation) scheme, a Vinegere ciphers, and so on - these can be trivially broken with today's computers. Don't use ``double DES'' (using DES twice) - that's subject to a ``man in the middle'' attack that triple-DES avoids. Your protocol should support multiple encryption algorithms, anyway; that way, when an encryption algorithm is broken, users can switch to another one.

For symmetric-key encryption (e.g., for bulk encryption), don't use a key length less than 90 bits if you want the information to stay secret through 2016 (add another bit for every additional 18 months of security) [Blaze 1996]. For encrypting worthless data, the old DES algorithm has some value, but with modern hardware it's too easy to break DES's 56-bit key using brute force. If you're using DES, don't just use the ASCII text key as the key - parity is in the least (not most) significant bit, so most DES algorithms will encrypt using a key value well-known to adversaries; instead, create a hash of the key and set the parity bits correctly (and pay attention to error reports from your encryption routine). So-called ``exportable'' encryption algorithms only have effective key lengths of 40 bits, and are essentially worthless; in 1996 an attacker could spend $10,000 to break such keys in twelve minutes or use idle computer time to break them in a few days, with the time-to-break halving every 18 months in either case.

Block encryption algorithms can be used in a number of different modes, such as ``electronic code book'' (ECB) and ``cipher block chaining'' (CBC). In nearly all cases, use CBC, and do not use ECB mode - in ECB mode, the same block of data always returns the same result inside a stream, and this is often enough to reveal what's encrypted. Many modes, including CBC mode, require an ``initialization vector'' (IV). The IV doesn't need to be secret, but it does need to be unpredictable by an attacker. Don't reuse IV's across sessions - use a new IV each time you start a session.

There are a number of different streaming encryption algorithms, but many of them have patent restrictions. I know of no patent or technical issues with WAKE. RC4 was a trade secret of RSA Data Security Inc; it's been leaked since, and I know of no real legal impediment to its use, but RSA Data Security has often threatened court action against users of it (it's not at all clear what RSA Data Security could do, but no doubt they could tie up users in worthless court cases). If you use RC4, use it as intended - in particular, always discard the first 256 bytes it generates, or you'll be vulnerable to attack. SEAL is patented by IBM - so don't use it. SOBER is patented; the patent owner has claimed that it will allow many uses for free if permission is requested, but this creates an impediment for later use. Even more interestingly, block encryption algorithms can be used in modes that turn them into stream ciphers, and users who want stream ciphers should consider this approach (you'll be able to choose between far more publicly-available algorithms).

11.5.3. Public Key Algorithms

For public key cryptography (used, among other things, for signing and sending secret keys), there are only a few widely-deployed algorithms. One of the most widely-used algorithms is RSA; RSA's algorithm was patented, but only in the U.S., and that patent expired in September 2000, so RSA can be freely used. Never decrypt or sign a raw value that an attacker gives you directly using RSA and expose the result, because that could expose the private key (this isn't a problem in practice, because most protocols involve signing a hash computed by the user - not the raw value - or don't expose the result). Never decrypt or sign the exact same raw value multiple times (the original can be exposed). Both of these can be solved by always adding random padding (PGP does this) - the usual approach is called Optimal Asymmetric Encryption Padding (OAEP).

The Diffie-Hellman key exchange algorithm is widely used to permit two parties to agree on a session key. By itself it doesn't guarantee that the parties are who they say they are, or that there is no middleman, but it does strongly help defend against passive listeners; its patent expired in 1997. If you use Diffie-Hellman to create a shared secret, be sure to hash it first (there's an attack if you use its shared value directly).

NIST developed the digital signature standard (DSS) (it's a modification of the ElGamal cryptosystem) for digital signature generation and verification; one of the conditions for its development was for it to be patent-free.

RSA, Diffie-Hellman, and El Gamal's techniques require more bits for the keys for equivalent security compared to typical symmetric keys; a 1024-bit key in these systems is supposed to be roughly equivalent to an 80-bit symmetric key. A 512-bit RSA key is considered completely unsafe; Nicko van Someren has demonstrated that such small RSA keys can be factored in 6 weeks using only already-available office hardware (never mind equipment designed for the job). In the past, a 1024-bit RSA key was considered reasonably secure, but recent advancements in factorization algorithms (e.g., by D. J. Bernstein) have raised concerns that perhaps even 1024 bits is not enough for an RSA key. Certainly, if your application needs to be highly secure or last beyond 2015, you should use a 2048 bit keys.

If you need a public key that requires far fewer bits (e.g., for a smartcard), then you might use elliptic curve cryptography (IEEE P1363 has some suggested curves; finding curves is hard). However, be careful - elliptic curve cryptography isn't patented, but certain speedup techniques are patented. Elliptic curve cryptography is fast enough that it really doesn't need these speedups anyway for its usual use of encrypting session / bulk encryption keys. In general, you shouldn't try to do bulk encryption with elliptic keys; symmetric algorithms are much faster and are better-tested for the job.

11.5.4. Cryptographic Hash Algorithms

Some programs need a one-way cryptographic hash algorithm, that is, a function that takes an ``arbitrary'' amount of data and generates a fixed-length number that hard for an attacker to invert (e.g., it's difficult for an attacker to create a different set of data to generate that same value). For a number of years MD5 has been a favorite, but recent efforts have shown that its 128-bit length may not be enough [van Oorschot 1994] and that certain attacks weaken MD5's protection [Dobbertin 1996]. Indeed, there are rumors that a top industry cryptographer has broken MD5, but is bound by employee agreement to keep silent (see the Bugtraq 22 August 2000 posting by John Viega). Anyone can create a rumor, but enough weaknesses have been found that the idea of completing the break is plausible. If you're writing new code, use SHA-1 instead of MD5. Don't use the original SHA (now called ``SHA-0''); SHA-0 had the same weakness that MD5 does. If you need more bits in your hash algorithm, use SHA-256, SHA-384, or SHA-512; you can get the specifications in NIST FIPS PUB 180-2.

11.5.5. Integrity Checking

When communicating, you need some sort of integrity check (don't depend just on encryption, since an attacker can then induce changes of information to ``random'' values). This can be done with hash algorithms, but don't just use a hash function directly (this exposes users to an ``extension'' attack - the attacker can use the hash value, add data of their choosing, and compute the new hash). The usual approach is ``HMAC'', which computes the integrity check as
  H(k xor opad, H(k xor ipad, data)).
where H is the hash function (typically MD5 or SHA-1) and k is the key. Thus, integrity checks are often HMAC-MD5 or HMAC-SHA-1. Note that although MD5 has some weaknesses, as far as I know MD5 isn't vulnerable when used in this construct, so HMAC-MD5 is (to my knowledge) okay. This is defined in detail in IETF RFC 2104.

Note that in the HMAC approach, a receiver can forge the same data as a sender. This isn't usually a problem, but if this must be avoided, then use public key methods and have the sender ``sign'' the data with the sender private key - this avoids this forging attack, but it's more expensive and for most environments isn't necessary.

11.5.6. Randomized Message Authentication Mode (RMAC)

NIST has developed and proposed a new mode for using cryptographic algorithms called Randomized Message Authentication Code (RMAC). RMAC is intended for use as a message authentication code technique.

Although there's a formal proof showing that RMAC is secure, the proof depends on the highly questionable assumption that the underlying cryptographic algorithm meets the "ideal cipher model" - in particular, that the algorithm is secure against a variety of specialized attacks, including related-key attacks. Unfortunately, related-key attacks are poorly studied for many algorithms; this is not the kind of property or attack that most people worry about when analyzing with cryptographic algorithms. It's known triple-DES doesn't have this properly, and it's unclear if other widely-accepted algorithms like AES have this property (it appears that AES is at least weaker against related key attacks than usual attacks).

The best advice right now is "don't use RMAC". There are other ways to do message authentication, such as HMAC combined with a cryptographic hash algorithm (e.g., HMAC-SHA1). HMAC isn't the same thing (e.g., technically it doesn't include a nonce, so you should rekey sooner), but the theoretical weaknesses of HMAC are merely theoretical, while the problems in RMAC seem far more important in the real world.

11.5.7. Other Cryptographic Issues

You should both encrypt and include integrity checks of data that's important. Don't depend on the encryption also providing integrity - an attacker may be able to change the bits into a different value, and although the attacker may not be able to change it to a specific value, merely changing the value may be enough. In general, you should use different keys for integrity and secrecy, to avoid certain subtle attacks.

One issue not discussed often enough is the problem of ``traffic analysis.'' That is, even if messages are encrypted and the encryption is not broken, an adversary may learn a great deal just from the encrypted messages. For example, if the presidents of two companies start exchanging many encrypted email messages, it may suggest that the two comparies are considering a merger. For another example, many SSH implementations have been found to have a weakness in exchanging passwords: observers could look at packets and determine the length (or length range) of the password, even if they couldn't determine the password itself. They could also also determine other information about the password that significantly aided in breaking it.

Be sure to not make it possible to solve a problem in parts, and use different keys when the trust environment (who is trusted) changes. Don't use the same key for too long - after a while, change the session key or password so an adversary will have to start over.

Generally you should compress something you'll encrypt - this does add a fixed header, which isn't so good, but it eliminates many patterns in the rest of the message as well as making the result smaller, so it's usually viewed as a ``win'' if compression is likely to make the result smaller.

In a related note, if you must create your own communication protocol, examine the problems of what's gone on before. Classics such as Bellovin [1989]'s review of security problems in the TCP/IP protocol suite might help you, as well as Bruce Schneier [1998] and Mudge's breaking of Microsoft's PPTP implementation and their follow-on work. Again, be sure to give any new protocol widespread public review, and reuse what you can.

11.6. Using PAM

Pluggable Authentication Modules (PAM) is a flexible mechanism for authenticating users. Many Unix-like systems support PAM, including Solaris, nearly all Linux distributions (e.g., Red Hat Linux, Caldera, and Debian as of version 2.2), and FreeBSD as of version 3.1. By using PAM, your program can be independent of the authentication scheme (passwords, SmartCards, etc.). Basically, your program calls PAM, which at run-time determines which ``authentication modules'' are required by checking the configuration set by the local system administrator. If you're writing a program that requires authentication (e.g., entering a password), you should include support for PAM. You can find out more about the Linux-PAM project at

11.7. Tools

Some tools may help you detect security problems before you field the result. They can't find all such problems, of course, but they can help catch problems that would overwise slip by. Here are a few tools, emphasizing open source / free software tools.

One obvious type of tool is a program to examine the source code to search for patterns of known potential security problems (e.g., calls to library functions in ways are often the source of security vulnerabilities). These kinds of programs are called ``source code scanners''. Here are a few such tools:

  • Flawfinder, which I've developed; it's available at This is also a program that scans C/C++ source code for common problems, and is also licensed under the GPL. Unlike RATS, flawfinder is implemented in Python. The developers of RATS and Flawfinder have agreed to find a way to work together to create a single ``best of breed'' open source program.

  • RATS (Rough Auditing Tool for Security) from Secure Software Solutions is available at This program scans C/C++ source code for common problems, and is licensed under the GPL.

  • ITS4 from Cigital (formerly Reliable Software Technologies, RST) also statically checks C/C++ code. It is available free for non-commercial use, including its source code and with certain modification and redistribution rights. Note that this isn't released as ``open source'' as defined by the Open Source Definition (OSD) - In particular, OSD point 6 forbids ``non-commercial use only'' clauses in open source licenses. ITS4 is available at

  • Splint (formerly named LCLint) is a tool for statically checking C programs. With minimal effort, splint can be used as a better lint. If additional effort is invested adding annotations to programs, splint can perform stronger checking than can be done by any standard lint. For example, it can be used to statically detect likely buffer overflows. The software is licensed under the GPL and is available at

  • cqual is a type-based analysis tool for finding bugs in C programs. cqual extends the type system of C with extra user-defined type qualifiers, e.g., it can note that values are ``tainted'' or ``untainted'' (similar to Perl's taint checking). The programmer annotates their program in a few places, and cqual performs qualifier inference to check whether the annotations are correct. cqual presents the analysis results using Program Analysis Mode, an emacs-based interface. The current version of cqual can detect potential format-string vulnerabilities in C programs. A previous incarnation of cqual, Carillon, has been used to find Y2K bugs in C programs. The software is licensed under the GPL and is available from

  • Cyclone is a C-like language intended to remove C's security weaknesses. In theory, you can always switch to a language that is ``more secure,'' but this doesn't always help (a language can help you avoid common mistakes but it can't read your mind). John Viega has reviewed Cyclone, and in December 2001 he said: ``Cyclone is definitely a neat language. It's a C dialect that doesn't feel like it's taking away any power, yet adds strong safety guarantees, along with numerous features that can be a real boon to programmers. Unfortunately, Cyclone isn't yet ready for prime time. Even with crippling limitations aside, it doesn't yet offer enough advantages over Java (or even C with a good set of tools) to make it worth the risk of using what is still a very young technology. Perhaps in a few years, Cyclone will mature into a robust, widely supported language that comes dangerously close to C in terms of efficiency. If that day comes, you'll certainly see me abandoning C for good.'' The Cyclone compiler has been released under the GPL and LGPL. You can get more information from the Cyclone web site.

Some tools try to detect potential security flaws at run-time, either to counter them or at least to warn the developer about them. Much of Crispen Cowan's work, such as StackGuard, fits here.

There are several tools that try to detect various C/C++ memory-management problems; these are really general-purpose software quality improvement tools, and not specific to security, but memory management problems can definitely cause security problems. An especially capable tool is Valgrind, which detects various memory-management problems (such as use of uninitialized memory, reading/writing memory after it's been free'd, reading/writing off the end of malloc'ed blocks, and memory leaks). Another such tool is Electric Fence (efence) by Bruce Perens, which can detect certain memory management errors. Memwatch (public domain) and YAMD (GPL) can detect memory allocation problems for C and C++. You can even use the built-in capabilities of the GNU C library's malloc library, which has the MALLOC_CHECK_ environment variable (see its manual page for more information). There are many others.

Another approach is to create test patterns and run the program, in attempt to find weaknesses in the program. Here are a few such tools:

  • BFBTester, the Brute Force Binary Tester, is licensed under the GPL. This program does quick security checks of binary programs. BFBTester performs checks of single and multiple argument command line overflows and environment variable overflows. Version 2.0 and higher can also watch for tempfile creation activity (to check for using unsafe tempfile names). At one time BFBTester didn't run on Linux (due to a technical issue in Linux's POSIX threads implementation), but this has been fixed as of version 2.0.1. More information is available at

  • The fuzz program is a tool for testing other software. It tests programs by bombarding the program being evaluated with random data. This tool isn't really specific to security.

  • SPIKE is a "fuzzer creation kit", i.e., it's a toolkit designed to create "random" tests to find security problems. The SPIKE toolkit is particularly designed for protocol analysis by simulating network protocol clients, and SPIKE proXy is a tool built on SPIKE to test web applications. SPIKE includes a few pre-canned tests. SPIKE is licensed under the GPL.

There are a number tools that try to give you insight into running programs that can also be useful when trying to find security problems in your code. This includes symbolic debuggers (such as gdb) and trace programs (such as strace and ltrace). One interesting program to support analysis of running code is Fenris (GPL license). Its documentation describes Fenris as a ``multipurpose tracer, stateful analyzer and partial decompiler intended to simplify bug tracking, security audits, code, algorithm or protocol analysis - providing a structural program trace, general information about internal constructions, execution path, memory operations, I/O, conditional expressions and much more.'' Fenris actually supplies a whole suite of tools, including extensive forensics capabilities and a nice debugging GUI for Linux. A list of other promising open source tools that can be suitable for debugging or code analysis is available at Another interesting program along these lines is Subterfugue, which allows you to control what happens in every system call made by a program.

If you're building a common kind of product where many standard potential flaws exist (like an ftp server or firewall), you might find standard security scanning tools useful. One good one is Nessus; there are many others. These kinds of tools are very useful for doing regression testing, but since they essentially use a list of past specific vulnerabilities and common configuration errors, they may not be very helpful in finding problems in new programs.

Often, you'll need to call on other tools to implement your secure infrastructure. The Open-Source PKI Book describes a number of open source programs for implmenting a public key infrastructure (PKI).

Of course, running a ``secure'' program on an insecure platform configuration makes little sense. You may want to examine hardening systems, which attempt to configure or modify systems to be more resistant to attacks. For Linux, one hardening system is Bastille Linux, available at

11.8. Windows CE

If you're securing a Windows CE Device, you should read Maricia Alforque's "Creating a Secure Windows CE Device" at

11.9. Write Audit Records

Write audit logs for program startup, session startup, and for suspicious activity. Possible information of value includes date, time, uid, euid, gid, egid, terminal information, process id, and command line values. You may find the function syslog(3) helpful for implementing audit logs. One awkward problem is that any logging system should be able to record a lot of information (since this information could be very helpful), yet if the information isn't handled carefully the information itself could be used to create an attack. After all, the attacker controls some of the input being sent to the program. When recording data sent by a possible attacker, identify a list of ``expected'' characters and escape any ``unexpected'' characters so that the log isn't corrupted. Not doing this can be a real problem; users may include characters such as control characters (especially NIL or end-of-line) that can cause real problems. For example, if an attacker embeds a newline, they can then forge log entries by following the newline with the desired log entry. Sadly, there doesn't seem to be a standard convention for escaping these characters. I'm partial to the URL escaping mechanism (%hh where hh is the hexadecimal value of the escaped byte) but there are others including the C convention (\ooo for the octal value and \X where X is a special symbol, e.g., \n for newline). There's also the caret-system (^I is control-I), though that doesn't handle byte values over 127 gracefully.

There is the danger that a user could create a denial-of-service attack (or at least stop auditing) by performing a very large number of events that cut an audit record until the system runs out of resources to store the records. One approach to counter to this threat is to rate-limit audit record recording; intentionally slow down the response rate if ``too many'' audit records are being cut. You could try to slow the response rate only to the suspected attacker, but in many situations a single attacker can masquerade as potentially many users.

Selecting what is ``suspicious activity'' is, of course, dependent on what the program does and its anticipated use. Any input that fails the filtering checks discussed earlier is certainly a candidate (e.g., containing NIL). Inputs that could not result from normal use should probably be logged, e.g., a CGI program where certain required fields are missing in suspicious ways. Any input with phrases like /etc/passwd or /etc/shadow or the like is very suspicious in many cases. Similarly, trying to access Windows ``registry'' files or .pwl files is very suspicious.

Do not record passwords in an audit record. Often people accidentally enter passwords for a different system, so recording a password may allow a system administrator to break into a different computer outside the administrator's domain.

11.10. Physical Emissions

Although it's really outside the scope of this book, it's important to remember that computing and communications equipment leaks a lot information that makes them hard to really secure. Many people are aware of TEMPEST requirements which deal with radio frequency emissions of computers, displays, keyboards, and other components which can be eavesdropped. The light from displays can also be eavesdropped, even if it's bounced off an office wall at great distance [Kuhn 2002]. Modem lights are also enough to determine the underlying communication.

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