CWE-135 Base Draft

Incorrect Calculation of Multi-Byte String Length

This vulnerability occurs when software incorrectly measures the length of strings containing multi-byte or wide characters, leading to buffer overflows, data corruption, or crashes.

Definition

What is CWE-135?

This vulnerability occurs when software incorrectly measures the length of strings containing multi-byte or wide characters, leading to buffer overflows, data corruption, or crashes.
Many programming languages and systems use multi-byte character encodings (like UTF-8) or wide characters (like UTF-16) where a single logical character can be made of multiple bytes. Standard string length functions (like strlen in C) often count bytes, not characters, which creates a mismatch. When security checks or memory allocations rely on this incorrect count, buffers can be overrun or under-read, creating a critical entry point for attacks. Developers encounter this when handling international text, file paths, or user input without using encoding-aware functions. To prevent it, always use dedicated library functions designed for your specific character encoding (like mbstowcs or wcslen for wide strings) and validate that memory operations account for the maximum possible bytes per character, not just the count of logical units.
Real-world impact

Real-world CVEs caused by CWE-135

No public CVE references are linked to this CWE in MITRE's catalog yet.

How attackers exploit it

Step-by-step attacker path

  1. 1

    Identify a code path that handles untrusted input without validation.

  2. 2

    Craft a payload that exercises the unsafe behavior — injection, traversal, overflow, or logic abuse.

  3. 3

    Deliver the payload through a normal request and observe the application's reaction.

  4. 4

    Iterate until the response leaks data, executes attacker code, or escalates privileges.

Vulnerable code example

Vulnerable C

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

Vulnerable C 
#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));
  		/* ... */
  }
Secure code example

Secure pseudo

Secure pseudo 
// Validate, sanitize, or use a safe API before reaching the sink.
function handleRequest(input) {
  const safe = validateAndEscape(input);
  return executeWithGuards(safe);
}
What changed: the unsafe sink is replaced (or the input is validated/escaped) so the same payload no longer triggers the weakness.
Prevention checklist

How to prevent CWE-135

  • Implementation Always verify the length of the string unit character.
  • Implementation Use length computing functions (e.g. strlen, wcslen, etc.) appropriately with their equivalent type (e.g.: byte, wchar_t, etc.)
Detection signals

How to detect CWE-135

Automated Static Analysis High

Automated static analysis, commonly referred to as Static Application Security Testing (SAST), can find some instances of this weakness by analyzing source code (or binary/compiled code) without having to execute it. Typically, this is done by building a model of data flow and control flow, then searching for potentially-vulnerable patterns that connect "sources" (origins of input) with "sinks" (destinations where the data interacts with external components, a lower layer such as the OS, etc.)

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Frequently asked questions

Frequently asked questions

What is CWE-135?

This vulnerability occurs when software incorrectly measures the length of strings containing multi-byte or wide characters, leading to buffer overflows, data corruption, or crashes.

How serious is CWE-135?

MITRE has not published a likelihood-of-exploit rating for this weakness. Treat it as medium-impact until your threat model proves otherwise.

What languages or platforms are affected by CWE-135?

MITRE lists the following affected platforms: C, C++.

How can I prevent CWE-135?

Always verify the length of the string unit character. Use length computing functions (e.g. strlen, wcslen, etc.) appropriately with their equivalent type (e.g.: byte, wchar_t, etc.)

How does Plexicus detect and fix CWE-135?

Plexicus's SAST engine matches the data-flow signature for CWE-135 on every commit. When a match is found, our Codex Remedium agent opens a fix PR with the corrected code, tests, and a one-line summary for the reviewer.

Where can I learn more about CWE-135?

MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/135.html. You can also reference OWASP and NIST documentation for adjacent guidance.

Related weaknesses

Weaknesses related to CWE-135

CWE-682 Parent

Incorrect Calculation

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CWE-128 Sibling

Wrap-around Error

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CWE-131 Sibling

Incorrect Calculation of Buffer Size

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CWE-1335 Sibling

Incorrect Bitwise Shift of Integer

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CWE-1339 Sibling

Insufficient Precision or Accuracy of a Real Number

This vulnerability occurs when a program uses a data type or algorithm that cannot accurately represent or calculate the fractional part…

CWE-190 Sibling

Integer Overflow or Wraparound

Integer overflow or wraparound occurs when a calculation produces a numeric result that exceeds the maximum value a variable can hold.…

CWE-191 Sibling

Integer Underflow (Wrap or Wraparound)

Integer underflow occurs when a subtraction operation results in a value smaller than the data type's minimum limit, causing the value to…

CWE-193 Sibling

Off-by-one Error

An off-by-one error occurs when a program incorrectly calculates a boundary, such as a loop counter or array index, by being one unit too…

CWE-369 Sibling

Divide By Zero

A divide-by-zero error occurs when software attempts to perform a division operation where the denominator is zero.

Resources

Further reading

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