STR32-C. Null-terminate byte strings as required

Null-terminated byte strings (NTBS) must contain a NULL-termination character at or before the address of the last element of the array before they can be safely passed as arguments to standard string-handling functions, such as strcpy() or strlen(). This is because these functions, as well as other string-handling functions defined by C99 [[ISO/IEC 9899-1999]], depend on the existence of a NULL-termination character to determine the length of a string. Similarly, NTBS must be NULL terminated before iterating on a character array where the termination condition of the loop depends on the existence of a NULL-termination character within the memory allocated for the string, as in the following example:

size_t i;
char ntbs[16];
/* ... */
for (i = 0; i < sizeof(ntbs); ++i) {
  if (ntbs[i] == '\0') break;
  /* ... */
}

Failure to properly terminate NULL-terminated byte strings can result in buffer overflows and other undefined behavior.

Non-Compliant Code Example (strncpy())

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.

In the first non-compliant coding example, ntbs is NULL terminated before the call to strncpy(). However, the subsequent execution of strncpy() may overwrite the NULL-termination character.

char ntbs[NTBS_SIZE];

ntbs[sizeof(ntbs)-1] = '\0';
strncpy(ntbs, source, sizeof(ntbs));

In the second non-compliant code example, memset() is used to clear the destination buffer; unfortunately, the third argument incorrectly specifies the size of the destination array [[Schwarz 05]].

char ntbs[NTBS_SIZE];

memset(ntbs, 0, sizeof(ntbs)-1);
strncpy(ntbs, source, sizeof(ntbs)-1);

Compliant Solution (Truncation)

The correct solution depends on the programmer's intent. If the intent was to truncate a string but ensure that the result remains a NULL-terminated string, this solution can be used:

char ntbs[NTBS_SIZE];
strncpy(ntbs, source, sizeof(ntbs)-1);
ntbs[sizeof(ntbs)-1] = '\0';

Compliant Solution (Copy without Truncation)

If the intent is to copy without truncation, this example copies the data and guarantee that the resulting NULL-terminated byte string is NULL terminated. If the string cannot be copied, it is handled as an error condition.

char *source = "0123456789abcdef";
char ntbs[NTBS_SIZE];
/* ... */
if (source) {
  if (strlen(source) < sizeof(ntbs)) {
    strcpy(ntbs, source);
  }
  else {
    /* handle string too large condition */
  }
}
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.

This compliant solution also guarantees that the string is NULL terminated.

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

Non-Compliant Code Example (realloc())

One method to decrease memory usage in critical situations when all available memory has been exhausted is to use the realloc() function to halve the size of message strings. The standard realloc() function has no concept of NULL-terminated byte strings. As a result, if realloc() is called to decrease the memory allocated for a NULL-terminated byte string, the NULL termination character may be truncated.

The following non-compliant code example fails to ensure that cur_msg is properly NULL terminated:

char *cur_msg = NULL;
size_t cur_msg_size = 1024;

/* ... */

void lessen_memory_usage(void) {
  char *temp;
  size_t temp_size;

  /* ... */

  if (cur_msg != NULL) {
    temp_size = cur_msg_size/2 + 1;
    temp = realloc(cur_msg, temp_size);
    if (temp == NULL) {
      /* Handle error condition */
    }
    cur_msg = temp;
    cur_msg_size = temp_size;
  }
}

/* ... */

Because realloc() does not guarantee that the string is properly NULL terminated, any subsequent operation on cur_msg that assumes a NULL-termination character may result in undefined behavior.

Compliant Solution (realloc())

In this compliant solution, the lessen_memory_usage() function ensures that the resulting string is always properly NULL terminated.

char *cur_msg = NULL;
size_t cur_msg_size = 1024;

/* ... */

void lessen_memory_usage(void) {
  char *temp;
  size_t temp_size;

  /* ... */

  if (cur_msg != NULL) {
    temp_size = cur_msg_size/2 + 1;
    temp = realloc(cur_msg, temp_size);
    if (temp == NULL) {
      /* Handle error condition */
    }
    cur_msg = temp;
    cur_msg_size = temp_size;

    /* ensure string is NULL-terminated */
    cur_msg[cur_msg_size - 1] = '\0';  
  }
}

/* ... */

Risk Assessment

Failure to properly NULL terminate strings can result in buffer overflows and the execution of arbitrary code with the permissions of the vulnerable process. Null-termination errors can also result in unintended information disclosure.

Related Vulnerabilities

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

References

[[ISO/IEC 9899-1999]] Section 7.1.1, "Definitions of terms," Section 7.20.3.4 "The realloc function," and Section 7.21, "String handling <string.h>"
[[ISO/IEC TR 24731-1-2007]] Section 6.7.1.4, "The strncpy_s function"
[[Schwarz 05]]
[[Seacord 05]] Chapter 2, "Strings"
[[Viega 05]] Section 5.2.14, "Miscalculated NULL termination"

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