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This code example is noncompliant because the character sequence c_str will not be null-terminated when passed as an argument to printf(). (see 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);
}

...

#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));
  return  ret = strlen(c_str);
  } else {
    /* Handle null pointer */
  }
  return ret;
}

Compliant Solution (Truncation)

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#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 return= strlen(c_str);
  } else {
    /* Handle null pointer */
  }
  return ret;
}

Compliant Solution (Truncation, strncpy_s())

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#define __STDC_WANT_LIB_EXT1__ 1
#include <string.h>

enum { STR_SIZE = 32 };

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

  if (source) {
    errno_t err = strncpy_s(
      ac_str, sizeof(ac_str), source, strlenstrnlen(source, sizeof(c_str))
    );
    if (err != 0) {
      /* Handle error */
    } else {
      ret = strnlen(c_str, sizeof(c_str));
    }
  } else {
     /* Handle null pointer */
  }
  return strlen_s(s, sizeof(a))ret;
}

Compliant Solution (Copy without Truncation)

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

Note that this code is not bulletproof. It gracefully handles the case where source  is NULL, when it is a valid string, and when source is not null-terminated, but at least the first 32 bytes are valid. However, in cases where source is not NULL, but points to invalid memory, or any of the first 32 bytes are invalid memory, the first call to strnlen() will access this invalid memory, and the resulting behavior is undefined. Unfortunately, standard C provides no way to prevent or even detect this condition without some external knowledge about the memory source points to.

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

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

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