In C, a string is just a buffer of characters, normally using the null character as a sentinel for the end of the string. This means
that the developer has to be aware of low-level details such as buffer sizes or having an extra character to store the final null
character. Doing that correctly and consistently is notoriously difficult and any error can lead to a security vulnerability, for instance, giving
access to sensitive data or allowing arbitrary code execution.
The function char *strncat( char *restrict dest, const char *restrict src, size_t count ); appends the characters of string
src at the end of dest, but only add count characters max. dest will always be
null-terminated. The wcsncat does the same for wide characters, and should be used with the same guidelines.
Ask Yourself Whether
- There is a possibility that either the
src or the dest pointer is null
- The current string length of
dest plus the current string length of src plus 1 (for the final null
character) is larger than the size of the buffer pointer-to by src
- There is a possibility that either string is not correctly
null-terminated
There is a risk if you answered yes to any of those questions.
Recommended Secure Coding Practices
- C11 provides, in its annex K, the
strncat_s and the wcsncat_s that were designed as safer alternatives to
strncat and wcsncat. It’s not recommended to use them in all circumstances because they introduce a runtime overhead and
require to write more code for error handling, but they perform checks that will limit the consequences of calling the function with bad arguments.
- Even if your compiler does not exactly support annex K, you probably have access to similar functions
- If you are using
strncat and wsncat as a safer version of strcat and wcscat, you should
instead consider strcat_s and wcscat_s because these functions have several shortcomings:
- It’s not easy to detect truncation
- The
count parameter is error-prone
- Computing the
count parameter typically requires computing the string length of dest, at which point other
simpler alternatives exist
Sensitive Code Example
int f(char *src) {
char dest[256];
strcpy(dest, "Result: ");
strncat(dest, src, sizeof dest); // Sensitive: passing the buffer size instead of the remaining size
return doSomethingWith(dest);
}
Compliant Solution
int f(char *src) {
char result[] = "Result: ";
char dest[256];
strcpy(dest, result);
strncat(dest, src, sizeof dest - sizeof result); // Compliant but may silently truncate
return doSomethingWith(dest);
}
See
