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Upon return, functions should guarantee that any objects object 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  Otherwise, type errors can occur 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 program may, as a result, process a string that it should not process, which might be a security flaw in itself. It may also cause the program to abort, which might be a denial-of-service attack. Also note that the emphasis is not to produce unterminated strings.

This The emphasis of this rule is to avoid producing unterminated string; it 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.

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The standard strncpy() function does not guarantee that the resulting string is null-terminated [ISO/IEC 9899:19992011]. 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 */
}

...

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 to enforce type safety can result in type errors in the program.

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CERT C++ Secure Coding Standard: API07-CPP. Enforce type safety

ISO/IEC 9899:19992011 Section 7.2124, "String handling <string.h>"

ISO/IEC TR 24772 "CJM String Terminationtermination"

ISO/IEC TR 24731-1:2007 Section 6.7.1.4, "The strncpy_s function"

Bibliography

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