Common mistakes in creating format strings include:

• using invalid conversion specifiers
• using a length modifier on an incorrect specifier
• mismatching the argument and conversion specifier type
• using invalid character classes

The following are C99 compliant conversion specifiers. Using any other specifier may result in undefined behavior.

d, i, o, u, x, X, f, F, e, E, g, G, a, A, c, s, p, n, %

Only some of the conversion specifiers are able to correctly take a length modifier. Using a length modifier on any specifier others than the following may result in undefined behavior.

d, i, o, u, x, X, a, A, e, E, f, F, g, G

Character class ranges must also be properly specified, with a hyphen in between two printable characters. The two following lines are both properly specified. The first accepts any character from a-z, inclusive, while the second accepts anything that is not a-z, inclusive.

[a-z]
[^a-z]

Mismatches between arguments and conversion specifiers may result in undefined behavior.

char *error_msg = "Resource not available to user.";
int error_type = 3;
/* ... */
printf("Error (type %s): %d\n", error_type, error_msg);

Risk Assessment

In most cases, incorrectly specified format strings will result in abnormal program termination.

Related Vulnerabilities

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

References

\[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] Section 7.19.6.1, "The {{fprintf}} function"

09. Input Output (FIO)      09. Input Output (FIO)       FIO01-A. Prefer functions that do not rely on file names for identification