8.4. Control Data Formatting (``Format Strings'')

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).

Surprisingly, many people seem to forget the power of these formatting capabilities and use data from untrusted users as the formatting parameter. Never use unfiltered data from an untrusted user as the format parameter. Perhaps this is best shown by example:

  /* Wrong ways: */
  /* Right ways: */
  printf("%s", string_from_untrusted_user); /* safe */
  fputs(string_from_untrusted_user); /* better for simple strings */

Otherwise, an attacker can cause all sorts of mischief by carefully selecting the formatting string. 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. 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; 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 and made vulnerable.

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 on permitting users to only select legal language values in Section 4.7.3

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.