Incorrect Calculation of Multi-Byte String Length

The software does not correctly calculate the length of strings that can contain wide or multi-byte characters.


The following examples help to illustrate the nature of this weakness and describe methods or techniques which can be used to mitigate the risk.

Note that the examples here are by no means exhaustive and any given weakness may have many subtle varieties, each of which may require different detection methods or runtime controls.

Example One

The following example would be exploitable if any of the commented incorrect malloc calls were used.

#include <stdio.h>
#include <strings.h>
#include <wchar.h>

int main() {

  wchar_t wideString[] = L"The spazzy orange tiger jumped " \
  "over the tawny jaguar.";
  wchar_t *newString;

  printf("Strlen() output: %d\nWcslen() output: %d\n",
  strlen(wideString), wcslen(wideString));

  /* Wrong because the number of chars in a string isn't related to its length in bytes //
  newString = (wchar_t *) malloc(strlen(wideString));

  /* Wrong because wide characters aren't 1 byte long! //
  newString = (wchar_t *) malloc(wcslen(wideString));

  /* Wrong because wcslen does not include the terminating null */
  newString = (wchar_t *) malloc(wcslen(wideString) * sizeof(wchar_t));

  /* correct! */
  newString = (wchar_t *) malloc((wcslen(wideString) + 1) * sizeof(wchar_t));

  /* ... */


The output from the printf() statement would be:

Strlen() output: 0
Wcslen() output: 53

See Also

SFP Secondary Cluster: Incorrect Buffer Length Computation

This category identifies Software Fault Patterns (SFPs) within the Incorrect Buffer Length Computation cluster (SFP10).

String Errors

Weaknesses in this category are related to the creation and modification of strings.

Comprehensive CWE Dictionary

This view (slice) covers all the elements in CWE.

CWE Cross-section

This view contains a selection of weaknesses that represent the variety of weaknesses that are captured in CWE, at a level of abstraction that is likely to be useful t...

Weaknesses Introduced During Implementation

This view (slice) lists weaknesses that can be introduced during implementation.

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