API07-C. Enforce type safety

Upon return, functions should guarantee that any objects returned by the function, or any modified value referenced by a pointer argument, is a valid object of function return type or argument type. Failure to do so can result in type errors in the program.

A good example is the null-terminated byte string type in C. If a string lacks the terminating null character, the program may be tricked into accessing storage after the string as legitimate data. This may cause a program to process a string that it should not, which might be a security flaw in itself. It may also cause the program to abort, which might be a denial of service attack.
Note also the emphasis is not to produce unterminated strings.

This does not address processing of already existing unterminated strings. However, by preventing the creation of unterminated strings, the need to process them is greatly lessened.

Noncompliant Code Example

The standard strncpy() function does not guarantee that the resulting string is null terminated [ISO/IEC 9899:1999]. If there is no null character in the first n characters of the source array, the result may not be null terminated.

char *source;
char a[NTBS_SIZE];
/* ... */
if (source) {
  errno_t err = strncpy(a, source, 5);
  if (err != 0) {
    /* Handle error */
  }
}
else {
  /* handle NULL string condition */
}

Compliant Solution (strncpy_s())

The strncpy_s() function copies up to n characters from the source array to a destination array [TR 24731]. If no null character was copied from the source array, then the nth position in the destination array is set to a null character, guaranteeing that the resulting string is null-terminated.

char *source;
char a[NTBS_SIZE];
/* ... */
if (source) {
  errno_t err = strncpy_s(a, sizeof(a), source, 5);
  if (err != 0) {
    /* Handle error */
  }
}
else {
  /* handle NULL string condition */
}

Risk Assessment

Failure to do so can result in type errors in the program.

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Other Languages

This rule appears in the C++ Secure Coding Standard as API07-CPP. Enforce type safety.

Bibliography

[ISO/IEC 9899:1999] Section 7.21, "String handling <string.h>"
[ISO/IEC PDTR 24772] "CJM String Termination"
[ISO/IEC TR 24731-1:2007] Section 6.7.1.4, "The strncpy_s function"

API04-C. Provide a consistent and usable error checking mechanism      13. Application Programming Interfaces (API)      API08-C. Protect header prototypes from misinterpretation