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 *strncpy(char * restrict dest, const char * restrict src, size_t count); copies the first count
characters from src to dest, stopping at the first null character, and filling extra space with 0. The
wcsncpy does the same for wide characters and should be used with the same guidelines.
Both of those functions are designed to work with fixed-length strings and might result in a non-null-terminated string.
Ask Yourself Whether
- There is a possibility that either the
source or the destination pointer is null
- The security of your system can be compromised if the
destination is a truncated version of the source
- The
source buffer can be both non-null-terminated and smaller than the count
- The
destination buffer can be smaller than the count
- You expect
dest to be a null-terminated string
- There is an overlap between the
source and the destination
There is a risk if you answered yes to any of those questions.
Recommended Secure Coding Practices
- C11 provides, in its annex K, the
strncpy_s and the wcsncpy_s that were designed as safer alternatives to
strcpy and wcscpy. 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
strncpy and wsncpy as a safer version of strcpy and wcscpy, you should
instead consider strcpy_s and wcscpy_s, because these functions have several shortcomings:
- It’s not easy to detect truncation
- Too much work is done to fill the buffer with 0, leading to suboptimal performance
- Unless manually corrected, the
dest string might not be null-terminated
- If you want to use
strcpy and wcscpy functions and detect if the string was truncated, the pattern is the following:
- Set the last character of the buffer to
null
- Call the function
- Check if the last character of the buffer is still
null
- If you are writing C++ code, using
std::string to manipulate strings is much simpler and less error-prone
Sensitive Code Example
int f(char *src) {
char dest[256];
strncpy(dest, src, sizeof(dest)); // Sensitive: might silently truncate
return doSomethingWith(dest);
}
Compliant Solution
int f(char *src) {
char dest[256];
dest[sizeof dest - 1] = 0;
strncpy(dest, src, sizeof(dest)); // Compliant
if (dest[sizeof dest - 1] != 0) {
// Handle error
}
return doSomethingWith(dest);
}
See
