Many library functions accept a string or wide string argument with the constraint that the string they receive is properly null-terminated. Passing a character sequence or wide character sequence that is not null-terminated to such a function can result in accessing memory that is outside the bounds of the object. Do not pass a character sequence or wide character sequence that is not null-terminated to a library function that expects a string or wide string argument. 

Noncompliant Code Example

This code example is noncompliant because the character sequence c_str will not be null-terminated when passed as an argument to printf(). (See STR11-C. Do not specify the bound of a character array initialized with a string literal on how to properly initialize character arrays.)

#include <stdio.h>
 
void func(void) {
  char c_str[3] = "abc";
  printf("%s\n", c_str);
}

Compliant Solution

This compliant solution does not specify the bound of the character array in the array declaration. If the array bound is omitted, the compiler allocates sufficient storage to store the entire string literal, including the terminating null character.

#include <stdio.h>
 
void func(void) {
  char c_str[] = "abc";
  printf("%s\n", c_str);
}

Noncompliant Code Example

This code example is noncompliant because the wide character sequence cur_msg will not be null-terminated when passed to wcslen(). This will occur if lessen_memory_usage() is invoked while cur_msg_size still has its initial value of 1024.

#include <stdlib.h>
#include <wchar.h>
 
wchar_t *cur_msg = NULL;
size_t cur_msg_size = 1024;
size_t cur_msg_len = 0;

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

  /* ... */

  if (cur_msg != NULL) {
    temp_size = cur_msg_size / 2 + 1;
    temp = realloc(cur_msg, temp_size * sizeof(wchar_t));
    /* temp &and cur_msg may no longer be null-terminated */
    if (temp == NULL) {
      /* Handle error */
    }

    cur_msg = temp;
    cur_msg_size = temp_size;
    cur_msg_len = wcslen(cur_msg); 
  }
}

Compliant Solution

In this compliant solution, cur_msg will always be null-terminated when passed to wcslen():

#include <stdlib.h>
#include <wchar.h>
 
wchar_t *cur_msg = NULL;
size_t cur_msg_size = 1024;
size_t cur_msg_len = 0;

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

  /* ... */

  if (cur_msg != NULL) {
    temp_size = cur_msg_size / 2 + 1;
    temp = realloc(cur_msg, temp_size * sizeof(wchar_t));
    /* temp and cur_msg may no longer be null-terminated */
    if (temp == NULL) {
      /* Handle error */
    }

    cur_msg = temp;
    /* Properly null-terminate cur_msg */
    cur_msg[temp_size - 1] = L'\0'; 
    cur_msg_size = temp_size;
    cur_msg_len = wcslen(cur_msg); 
  }
}

Noncompliant Code Example (strncpy())

Although the strncpy() function takes a string as input, it does not guarantee that the resulting value is still null-terminated. In the following noncompliant code example, if no null character is contained in the first n characters of the source array, the result will not be null-terminated. Passing a non-null-terminated character sequence to strlen() is undefined behavior.

#include <string.h>
 
enum { STR_SIZE = 32 };
 
size_t func(const char *source) {
  char c_str[STR_SIZE];
  size_t ret = 0;

  if (source) {
    c_str[sizeof(c_str) - 1] = '\0';
    strncpy(c_str, source, sizeof(c_str));
    ret = strlen(c_str);
  } else {
    /* Handle null pointer */
  }
  return ret;
}

Compliant Solution (Truncation)

This compliant solution is correct if the programmer's intent is to truncate the string:

#include <string.h>
 
enum { STR_SIZE = 32 };
 
size_t func(const char *source) {
  char c_str[STR_SIZE];
  size_t ret = 0;

  if (source) {
    strncpy(c_str, source, sizeof(c_str) - 1);
    c_str[sizeof(c_str) - 1] = '\0';
    ret = strlen(c_str);
  } else {
    /* Handle null pointer */
  }
  return ret;
}

Compliant Solution (Truncation, strncpy_s())

The C Standard, Annex K strncpy_s() function can also be used to copy with truncation. The strncpy_s() function copies up to n characters from the source array to a destination array. 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.

#define __STDC_WANT_LIB_EXT1__ 1
#include <string.h>

enum { STR_SIZE = 32 };

size_t func(const char *source) {
  char a[STR_SIZE];
  size_t ret = 0;

  if (source) {
    errno_t err = strncpy_s(
      a, sizeof(a), source, strlen(source)
    );
    if (err != 0) {
      /* Handle error */
    } else {
      ret = strnlen_s(a, sizeof(a));
    }
  } else {
     /* Handle null pointer */
  }
  return ret;
}

Compliant Solution (Copy without Truncation)

If the programmer's intent is to copy without truncation, this compliant solution copies the data and guarantees that the resulting array is null-terminated. If the string cannot be copied, it is handled as an error condition.

#include <string.h>
 
enum { STR_SIZE = 32 };
 
size_t func(const char *source) {
  char c_str[STR_SIZE];
  size_t ret = 0;

  if (source) {
    if (strlen(source) < sizeof(c_str)) {
      strcpy(c_str, source);
      ret = strlen(c_str);
    } else {
      /* Handle string-too-large */
    }
  } else {
    /* Handle null pointer */
  }
  return ret;
}

Risk Assessment

Failure to properly null-terminate a character sequence that is passed to a library function that expects a string 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.

Automated Detection

Tool	Version	Checker	Description
Astrée			Supported Astrée supports the implementation of library stubs to fully verify this guideline.
Axivion Bauhaus Suite		CertC-STR32	Partially implemented: can detect some violation of the rule
CodeSonar		MISC.MEM.NTERM.CSTRING	Unterminated C String
Compass/ROSE			Can detect some violations of this rule
Coverity		STRING_NULL	Fully implemented
Klocwork		NNTS.MIGHT NNTS.MUST NNTS.TAINTED
LDRA tool suite		404 S, 600 S	Partially implemented
Parasoft C/C++test		CERT_C-STR32-a	Avoid overflow due to reading a not zero terminated string
Polyspace Bug Finder		Invalid use of standard library string routine Standard function call with incorrect arguments Tainted NULL or non-null-terminated string MISRA C:2012 Rule 21.17	Standard library string function called with invalid arguments Argument to a standard function does not meet requirements for use in the function Argument is from an unsecure source and may be NULL or not NULL-terminated Use of the string handling function from <string.h> shall not result in accesses beyond the bounds of the objects referenced by their pointer parameters
PRQA QA-C		2835, 2836, 2839
PRQA QA-C++		0145
PVS-Studio		V692
TrustInSoft Analyzer		match format and arguments	Partially verified.

Related Vulnerabilities

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

Related Guidelines

Key here (explains table format and definitions)

CERT-CWE Mapping Notes

Key here for mapping notes

CWE-119 and STR32-C

Independent( ARR30-C, ARR38-C, ARR32-C, INT30-C, INT31-C, EXP39-C, EXP33-C, FIO37-C) STR31-C = Subset( Union( ARR30-C, ARR38-C)) STR32-C = Subset( ARR38-C)

CWE-119 = Union( STR32-C, list) where list =

• Out-of-bounds reads or writes that do not involve non-null-terminated byte strings.

CWE-125 and STR32-C

Independent( ARR30-C, ARR38-C, EXP39-C, INT30-C) STR31-C = Subset( Union( ARR30-C, ARR38-C)) STR32-C = Subset( ARR38-C)

CWE-125 = Union( STR32-C, list) where list =

• Out-of-bounds reads that do not involve non-null-terminated byte strings.

CWE-123 and STR32-C

Independent(ARR30-C, ARR38-C) STR31-C = Subset( Union( ARR30-C, ARR38-C)) STR32-C = Subset( ARR38-C)

Intersection( CWE-123, STR32-C) =

• Buffer overflow from passing a non-null-terminated byte string to a standard C library copying function that expects null termination, and that overwrites an (unrelated) pointer

STR32-C - CWE-123 =

• Buffer overflow from passing a non-null-terminated byte string to a standard C library copying function that expects null termination, but it does not overwrite an (unrelated) pointer

CWE-123 – STR31-C =

• Arbitrary writes that do not involve standard C library copying functions, such as strcpy()

Bibliography