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The formatted output functions (fprintf() and related functions) convert, format, and print their arguments under control of a format string, defined by the C Standard, 7.21.6.1, paragraph 3 [ISO/IEC 9899:2011], specifies:

The format shall be a multibyte character sequence, beginning and ending in its initial shift state. The format is composed of zero or more directives: ordinary multibyte characters (not %), which are copied unchanged to the output stream; and conversion specifications, each of which results in fetching zero or more subsequent arguments, converting them, if applicable, according to the corresponding conversion specifier, and then writing the result to the output stream.

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The following table summarizes the compliance of various conversion specifications. The first column contains a one or more conversion specifier character (or characters). The next four columns consider the combination of the specifier character(s) characters with the various flags (the apostrophe ['], -, +, the space character, #, and 0). The next eight columns consider the combination of the specifier character(s) characters with the various length modifiers (h, hh, l, ll, j, z, t, and L). Valid combinations are marked with a type name; arguments matched with the conversion specification will be are interpreted as that type. For example, an argument matched with the specifier %hd will be is interpreted as a short, so short appears in the cell where d and h intersect. The last column denotes the expected type of arguments matched with the original specifier character(s)characters. Valid and meaningful combinations are marked by the  symbol (save for the length modifier columns, as described abovepreviously). Valid combinations that have no effect are labeled N/E. Using a combination marked by the symbol, using a specification not represented in the table, or using an argument of an unexpected type may result in is undefined behavior. See (see undefined behaviors 153, 155, 157, 158, 161, and 162 in Annex J of the C Standard). 

Legend     SPACE:

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The space (" ") character
     N/E

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: No effect

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     NTBS: char* argument pointing to a null-terminated

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character string

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     NTWS: wchar_t* argument pointing to a null-terminated wide character string

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     XSI: ISO/IEC 9945-2003 XSI extension

The formatted input functions (fscanf() and related functions) use similarly - specified format strings and impose similar restrictions on their format strings and arguments.

Do not supply an unknown or invalid conversion specification or an invalid combination of flag character, precision, length modifier, conversion specifier ; to a formatted IO function. Likewise, do not provide a number or type of arguments that do not match the argument type of the conversion specifier used in the format string.

Format strings are usually string literals specified at the call site, but they need not be. They should, however, not contain unsanitized data; However, they should not contain tainted values (see FIO30-C. Exclude user input from format strings for more information).

Noncompliant Code Example

Mismatches between arguments and conversion specifications may result in undefined behavior. Compilers may diagnose type mismatches in formatted output function invocations. In the following this noncompliant code example, the error_type argument to printf() is incorrectly matched with the %s s specifier , rather than with the %d d specifier. Likewise, the error_msg argument is incorrectly matched with the %d d specifier instead of the %sthe s specifier. These usages result in undefined behavior. One possible result of this invocation is that printf() will interpret the error_type argument as a pointer , and try to read a string from the address that error_type contains, possibly resulting in an access violation.

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This compliant solution ensures that the format arguments to the printf() function match their respective format conversion specifications:

#include <stdio.h>
 
void func(void) {
  const char *error_msg = "Resource not available to user.";
  int error_type = 3;
  /* ... */
  printf("Error (type %d): %s\n", error_type, error_msg);

  /* ... */
}

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Incorrectly specified format strings can result in memory corruption or abnormal program termination. However, in some cases they can be used to corrupt memory in a manner controllable by an attacker.

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Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

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Bibliography

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