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 *strcpy(char * restrict dest, const char * restrict src); copies characters from src to
dest. The wcscpy does the same for wide characters and should be used with the same guidelines.
Note: the functions strncpy and wcsncpy might look like attractive safe replacements for strcpy and
wcscpy, but they have their own set of issues (see S5816), and you should probably prefer another more adapted
alternative.
Ask Yourself Whether
- There is a possibility that either the source or the destination pointer is
null
- There is a possibility that the source string is not correctly
null-terminated, or that its length (including the final
null character) can be larger than the size of the destination buffer.
- There is an overlap between source and 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
strcpy_s and the wcscpy_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, for example,
strlcpy in
FreeBSD
- 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];
strcpy(dest, src); // Sensitive: might overflow
return doSomethingWith(dest);
}
Compliant Solution
int f(char *src) {
char *dest = malloc(strlen(src) + 1); // For the final 0
strcpy(dest, src); // Compliant: we made sure the buffer is large enough
int r= doSomethingWith(dest);
free(dest);
return r;
}
See
