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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 as follows by the . The C Standard, 7.2123.6.1, paragraph 3 [ISO/IEC 9899:20112024]:, 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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• 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, the maximum number of digits to appear for certain conversion specifiers, or the maximum number of bytes, etc. depending on the conversion specifier
• 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 insufficient 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 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).  Here, valid

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 types of arguments matched with the original specifier characters.

Valid character(s).  Throughout the table, 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 undefined behaviors 153, 155, 157, 158, 161, and 162 in Annex J of the C Standard.) 

Conversion Specifier Character	' XSI	- + SPACE	#	0

 

	h	hh	l	ll	j	z	t	L	Argument Type
d, i

 

short	signed char	long	long long	intmax_t	size_t	ptrdiff_t		Signed integer
o

 

unsigned short	unsigned char	unsigned long	unsigned long long	uintmax_t	size_t	ptrdiff_t		Unsigned integer
u

 

	unsigned short	unsigned  char	unsigned long	unsigned long long	uintmax_t	size_t	ptrdiff_t		Unsigned integer
x, X

 

	unsigned short	unsigned char	unsigned long	unsigned long long	uintmax_t	size_t	ptrdiff_t		Unsigned integer
f, F

 

			N/E	N/E				long double	double or long double
e, E

 

			N/E	N/E				long double	double or long double
g, G

 

			N/E	N/E				long double	double or long double
a, A

 

			N/E	N/E				long double	double or long double
c

 

			wint_t						int or wint_t
s

 

			NTWS						NTBS or NTWS
p

 

									void*
n

 

short*	char*	long*	long long*	intmax_t*	size_t*	ptrdiff_t*		Pointer to integer
C XSI

 

								wint_t
S XSI

 

									NTWS
%

 

None

Legend     SPACE:

...

The space (" ") character
     N/E

...

: No effect

...

     NTBS: char* argument pointing to a null-terminated

...

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 format 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 arguments argument that do does not match the argument type of the conversion specifiers 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; see 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.   Many compilers can 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 specifierthe d specifier. Likewise, the 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. This is likely to result , 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);
  /* ... */
}

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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);

  /* ... */
}

Risk Assessment

In most cases, incorrectly Incorrectly specified format strings will can result in memory corruption or abnormal program termination. However, in some cases they can be used to corrupt memory in manners controllable by an attacker.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Key here (explains table format and definitions)

CERT-CWE Mapping Notes

Key here for mapping notes

CWE-686 and FIO47-C

Intersection( EXP37-C, FIO47-C) =

• Invalid argument types passed to format I/O function

EXP37-C – FIO47-C =

• Invalid argument types passed to non-format I/O function

FIO47-C – EXP37-C =

• Invalid format string, but correctly matches arguments in number and type

Intersection( CWE-686, FIO47-C) =

• Use of format strings that do not match the type of arguments

CWE-686 – FIO47-C =

• Incorrect argument type in functions outside of the printf() family.

FIO47-C – CWE-686 =

• Invalid format strings that still match their arguments in type

CWE-685 and FIO47-C

Intersection( CWE-685, FIO47-C) =

• Use of format strings that do not match the number of arguments

CWE-685 – FIO47-C =

• Incorrect argument number in functions outside of the printf() family.

FIO47-C – CWE-685 =

• Invalid format strings that still match their arguments in number

CWE-134 and FIO47-C

Intersection( FIO30-C, FIO47-C) =

• Use of untrusted and ill-specified format string

FIO30-C – FIO47-C =

• Use of untrusted, but well-defined format string

FIO47-C – FIO30-C =

• Use of Ill-defined, but trusted format string

FIO47-C = Union(CWE-134, list) where list =

• Using a trusted but invalid format string

Bibliography

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