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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.

Rule

Severity

Likelihood

Remediation Cost

Priority

Level

STR32-C

High

Probable

Medium

P12

L1

Automated Detection

Tool

Version

Checker

Description

Astrée
19.04

Supported

Astrée supports the implementation of library stubs to fully verify this guideline.

Axivion Bauhaus Suite

6.9.0

CertC-STR32Partially implemented: can detect some violation of the rule
CodeSonar
5.1p0
MISC.MEM.NTERM.CSTRINGUnterminated C String
Compass/ROSE



Can detect some violations of this rule

Coverity
2017.07
STRING_NULLFully implemented
Klocwork
2018

NNTS.MIGHT
NNTS.MUST

NNTS.TAINTED


LDRA tool suite
9.7.1

404 S, 600 S

Partially implemented

Parasoft C/C++test
10.4.2
CERT_C-STR32-a

Avoid overflow due to reading a not zero terminated string

Polyspace Bug Finder

R2019b

CERT C: Rule STR32-C


Checks for:

  • Invalid use of standard library string routine
  • Tainted NULL or non-null-terminated string

Rule partially covered.

PRQA QA-C

9.5

2835, 2836, 2839
PRQA QA-C++

4.3

0145 
PVS-Studio

6.23

V692
TrustInSoft Analyzer

1.38

match format and argumentsPartially 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)

Taxonomy

Taxonomy item

Relationship

ISO/IEC TR 24772:2013String Termination [CMJ]Prior to 2018-01-12: CERT: Unspecified Relationship
ISO/IEC TS 17961:2013Passing a non-null-terminated character sequence to a library function that expects a string [strmod]Prior to 2018-01-12: CERT: Unspecified Relationship
CWE 2.11CWE-119, Improper Restriction of Operations within the Bounds of a Memory Buffer2017-05-18: CERT: Rule subset of CWE
CWE 2.11CWE-123, Write-what-where Condition2017-06-12: CERT: Partial overlap
CWE 2.11CWE-125, Out-of-bounds Read2017-05-18: CERT: Rule subset of CWE
CWE 2.11CWE-170, Improper Null Termination2017-06-13: CERT: Exact

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

[Seacord 2013] Chapter 2, "Strings" 
[Viega 2005]Section 5.2.14, "Miscalculated NULL Termination"






3 Comments

  1. > This only occurs if the index is less than the minimum size of the string.

    ITYM "maximum", no?

  2. I replaced the realloc() example with a similar example from TS 17961 [nonnullcs] that uses wchar_t. 

    Rewrote the rule intro as well.  TS 17961 [nonnullcs] omits the strncpy() example.  I'm wondering if we should follow suite?

     

  3. I suggest a third compliant solution (with truncation) that uses the function strtcpy (t means truncation allowed).

    The prototype of strtcpy is:    

         int   strtcpy( char * dst, size_t dstsize, const char * src) ;

    This function (in fact, it is a macro) copies the string src in the array dst where dstsize is the size of dst. It returns a negative value if dst or src is the NULL pointer or if dstsize is 0.

    The return value distinguishes if a truncation happened or not.

    #include <string.h>

    #include “str5.h”

    enum { STR_SIZE = 32 };

     

    size_t func(const char *source) {

      char c_str[STR_SIZE];

      size_t ret = 0;

        int err ;

     

      err = strtcpy(c_str, sizeof(c_str), source) ;

      if ( err < 0 ) {

        /* Handle error */

      } else {  

        ret = strlen(c_str);

      }

      return ret;

    }

     

    The function strtcpy is easier to use than strncpy_s and more robust than strncpy or strlcpy.

    Like strlcpy, strtcpy is C90 compliant, open-source (same licence than glibc), easy to download and to include in C projects.

    [see http://aral.iut-rodez.fr/en/sanchis/miscellaneous/str5/str5.html  and  str5.h]