STR38-C. Do not use wide-char functions on narrow-char strings and vice versa

Wide characters can frequently contain null bytes if taken from the ASCII character set. As a result, using narrow-character functions that rely on null-byte termination can lead to obtuse behavior. Likewise, a narrow-character string that is properly null-terminated might not be considered so in a wide-character function. Improper use of narrow and wide character strings could result in buffer overflows.

Noncompliant Code Example (Using strncpy instead of wcsncpy)

This example uses strncpy, which copies, at most, 10 bytes but will stop copying after it encounters a null-byte. Because wide characters can contain null-bytes, the code can stop copying prematurely. It is important to recognize that many narrow-string functions are byte functions and so can terminate prematurely.

wchar_t wide_str1[]  = L"0123456789";
wchar_t wide_str2[] =  L"0000000000";

strncpy(wide_str2, wide_str1, 10);

Noncompliant Code Example (Using wcsncpy instead of strncpy)

In most implementations, wide characters span multiple narrow characters. The wcsncpy function copies, at most, 10 wide characters, which is longer than narrow_str1. As a result, it will write the first 10 bytes of narrow_str1 into narrow_str2 and then continue padding with L'\0' null wide characters until 10 wide characters have been written.

Note that wcsncpy does not perform null-termination if the source string contains more wide characters than the destination. As a result, it is possible for an attacker to exploit such a vulnerability by passing a maliciously crafted string to wcsncpy. If the code is intended to copy a certain number of bytes, it can overflow the buffer by writing multiple bytes because wcsncpy measures copying by wide characters, not by bytes.

char narrow_str1[] = "0123456789";
char narrow_str2[] = "0000000000";

wcsncpy(narrow_str2, narrow_str1, 10);

Implementation Details

The C standard recognizes wchar_t[] and char[] as distinct types. As a result, many compilers will yield a warning if the inappropriate function is used. For example, the following warnings were generated when the second noncompliant example was compiled with no flags in GCC on a Linux i686 platform:

warning: passing arg 1 of `wcsncpy' from incompatible pointer type
warning: passing arg 2 of `wcsncpy' from incompatible pointer type

Similar warnings were issued by the compiler for the first noncompliant example, with respect to the arguments of the strncpy function instead.

Since these are just warnings, the compiled code can still be run. When run on the i686 Linux platform, both noncompliant code examples began copying information from out of the bounds of the arguments. This behavior indicates a possible buffer overflow vulnerability.

Compliant Solution

This compliant solution uses the appropriate-width function versions. Using wcsncpy for wide-character strings and strncpy for narrow-character strings ensures that data is not truncated or overwriting extra memory.

wchar_t wide_str1[] = L"0123456789";
wchar_t wide_str2[] = L"0000000000";
wcsncpy(wide_str2, wide_str1, 10);   /* Use of proper-width function */

char narrow_str1[] = "0123456789";
char narrow_str2[] = "0000000000";
strncpy(narrow_str2, narrow_str1, 10); /* Use of proper-width function */

Risk Assessment

Failure to use the proper-width string functions can lead to buffer overflows and the execution of arbitrary code by an attacker.

Automated Detection

Modern compilers recognize the difference between a char* and a wchar_t* pointer. As a result, compiling code that violates this rule will generate warnings. It is feasible to have automated software that recognizes functions of improper width and replaces them with functions of proper width (that is, software that uses wcsncpy when it recognizes that the parameters are of type wchar_t*).

Related Vulnerabilities

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

Related Guidelines

ISO/IEC 9899:2011 Section 7.24.2.4, "The strncpy function," and Section 7.29.4.2.2, "The wcsncpy function"

Bibliography