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 */
Noncompliant Code Example (strlen()
)
In this noncompliant code example, the strlen()
function is used to determine the size of a wide character string:
wchar_t wide_str1[] = L"0123456789"; wchar_t *wide_str2 = (wchar_t *)malloc(strlen(wide_str1) + 1); if (wide_str2 == NULL) { /* Handle error */ } /* ... */ free(wide_str2); wide_str2 = NULL;
The strlen()
function counts the number of characters in a null-terminated byte string preceding the terminating null byte. However, wide characters contain null bytes, particularly when taken from the ASCII character set, as in this example. As a result, the strlen()
function will return the number of bytes preceding the first null byte in the string.
Compliant Solution
This compliant solution correctly calculates the number of bytes required to contain a copy of the wide string (including the termination character):
wchar_t wide_str1[] = L"0123456789"; wchar_t *wide_str2 = (wchar_t *)malloc( (wcslen(wide_str1) + 1) * sizeof(wchar_t) ); if (wide_str2 == NULL) { /* Handle error */ } /* ... */ free(wide_str2); wide_str2 = NULL;
Risk Assessment
Failure to use the proper-width string functions can lead to buffer overflows and the execution of arbitrary code by an attacker.
Rule | Severity | Likelihood | Remediation Cost | Priority | Level |
---|---|---|---|---|---|
STR38-C | high | likely | low | P27 | L1 |
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.