Using "strcat" or "wcscat" is security-sensitive

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 *strcat( char *restrict dest, const char *restrict src ); appends the characters of string src at the end of dest. The wcscat does the same for wide characters and should be used with the same guidelines.

Note: the functions strncat and wcsncat might look like attractive safe replacements for strcat and wcscaty, but they have their own set of issues (see S5815), and you should probably prefer another more adapted alternative.

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 strcat_s and the wcscat_s that were designed as safer alternatives to strcat and wcscat. 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 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, "Result: ");
  strcat(dest, src); // Sensitive: might overflow
  return doSomethingWith(dest);
}

Compliant Solution

int f(char *src) {
  char result[] = "Result: ";
  char *dest = malloc(sizeof(result) + strlen(src)); // Not need of +1 for final 0 because sizeof will already count one 0
  strcpy(dest, result);
  strcat(dest, src); // Compliant: the buffer size was carefully crafted
  int r = doSomethingWith(dest);
  free(dest);
  return r;
}

See

• OWASP Top 10 2017 Category A9 - Using Components with Known Vulnerabilities
• MITRE, CWE-120 - Buffer Copy without Checking Size of Input ('Classic Buffer Overflow')
• CERT, STR07-C. - Use the bounds-checking interfaces for string manipulation