FIO47-C. Use valid format strings

The formatted output functions (fprintf() and related functions) convert, format, and print their arguments under control of a format string. 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.

Each conversion specification is introduced by the % character followed (in order) by

• Zero or more flags (in any order), which modify the meaning of the conversion specification
• An optional minimum field width
• An optional precision that gives the minimum number of digits to appear for certain conversion specifiers
• An optional length modifier that specifies the size of the argument
• A conversion specifier character that indicates the type of conversion to be applied

Common mistakes in creating format strings include

• Providing an incorrect number of arguments for the format string
• Using invalid conversion specifiers
• Using a flag character that is incompatible with the conversion specifier
• Using a length modifier that is incompatible with the conversion specifier
• Mismatching the argument type and conversion specifier
• Using an argument of type other than int for width or precision

The following table summarizes the compliance of various conversion specifications. The first column contains one or more conversion specifier characters. The next four columns consider the combination of the specifier characters with the various flags (the apostrophe ['], -, +, the space character, #, and 0). The next eight columns consider the combination of the specifier 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 are interpreted as that type. For example, an argument matched with the specifier %hd is interpreted as a short, so short appears in the cell where d and h intersect. The last column denotes the expected types of arguments matched with the original specifier characters.

Valid and meaningful combinations are marked by the  symbol (save for the length modifier columns, as described previously). 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 is undefined behavior. (See undefined behaviors 153, 155, 157, 158, 161, and 162.) 

     SPACE: The space (" ") character
     N/E: No effect
     NTBS: char* argument pointing to a null-terminated character string
     NTWS: wchar_t* argument pointing to a null-terminated wide character string
     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, or conversion specifier to a formatted IO function. Likewise, do not provide a number or type of argument that does 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. 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 this noncompliant code example, the error_type argument to printf() is incorrectly matched with the s specifier rather than with the d specifier. Likewise, the error_msg argument is incorrectly matched with the d specifier instead of the 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.

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

Compliant Solution

This compliant solution ensures that the arguments to the printf() function match their respective 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);

  /* ... */
}

Risk Assessment

Incorrectly specified format strings can result in memory corruption or abnormal program termination.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Bibliography