STR32-C. Do not pass a non-null-terminated character sequence to a library function that expects a string

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 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.

#include <stdio.h>
 
void func(void) {
  char str[3] = "abc";
  printf("%s\n", 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 str[] = "abc";
  printf("%s\n", 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 & cur_msg might not 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 & cur_msg might not be null-terminated
    if (temp == NULL) {
      /* Handle error */
    }

    cur_msg = temp;
    // cur_msg now properly null-terminated
    cur_msg[temp_size - 1] = L'\0'; 
    cur_msg_size = temp_size;
    cur_msg_len = wcslen(cur_msg); 
  }
}

Noncompliant Code Example (strncpy())

While 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() results in undefined behavior:

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

  str[sizeof(str) - 1] = '\0';
  strncpy(str, source, sizeof(str));
  return strlen(str);
}

Compliant Solution (Truncation)

The correct solution depends on the programmer's intent. If the intent is to truncate a string, this solution can be used:

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

  strncpy(str, source, sizeof(str) - 1);
  str[sizeof(str) - 1] = '\0';
  return strlen( str);
}

Compliant Solution (Copy without Truncation)

If the intent is to copy without truncation, this example 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 str[STR_SIZE];

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

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. 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];

  if (source) {
    errno_t err = strncpy_s(a, sizeof(a), source, 5);
    if (err != 0) {
      /* Handle error */
    }
  } else {
    /* Handle null pointer */
  }
  return strlen_s(s, sizeof(a));
}

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

Bibliography