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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. A program may, as a result, process a string 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.

The emphasis of this rule recommendation is to avoid producing unterminated stringstrings; 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 9899:2011]. 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) {
  char* b = strncpy(a, source, 5); // b == a
}
else {
  /* handleHandle NULLnull string condition */
}

Compliant Solution (strncpy_s(), C11 Annex K)

The C11 Annex K 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, 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 {
  /* handleHandle NULLnull string condition */
}

Risk Assessment

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

Automated Detection

Related Vulnerabilities

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

Related Guidelines

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Key here (explains table format and definitions)

Taxonomy	Taxonomy item	Relationship
ISO/IEC TR 24772

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:2013	String Termination [CJM]	Prior to 2018-01-12: CERT: Unspecified Relationship
MITRE CWE	CWE-192	Prior to 2018-01-12:
MITRE CWE	CWE-227	Prior to 2018-01-12:
MITRE CWE	CWE-590	Prior to 2018-01-12:
MITRE CWE	CWE-686	Prior to 2018-01-12:
MITRE CWE	CWE-704	Prior to 2018-01-12:
MITRE CWE	CWE-761	Prior to 2018-01-12:
MITRE CWE	CWE-762	Prior to 2018-01-12:
MITRE CWE	CWE-843	Prior to 2018-01-12:

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Image Added Image Added API09-C. Compatible values should have the same type

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

Sources

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