CWE-135 Base Rascunho

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.

Definição

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.
Impacto no mundo real

Real-world CVEs caused by CWE-135

Ainda não há referências CVE públicas associadas a este CWE no catálogo da MITRE.

Como os atacantes a exploram

Trajeto do atacante passo a passo

  1. 1

    Identificar um caminho de código que trata input não confiável sem validação.

  2. 2

    Criar um payload que explora o comportamento inseguro — injeção, traversal, overflow ou abuso de lógica.

  3. 3

    Entregar o payload através de um pedido normal e observar a reação da aplicação.

  4. 4

    Iterar até que a resposta exponha dados, execute código do atacante ou escale privilégios.

Exemplo de código vulnerável

Vulnerable C

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

Vulnerável 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));
  		/* ... */
  }
Exemplo de código seguro

Secure pseudo

Seguro 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.
Lista de verificação de prevenção

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.)
Sinais de deteção

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.)

Correção automática do Plexicus

O Plexicus deteta automaticamente o CWE-135 e abre um PR de correção em menos de 60 segundos.

O Codex Remedium analisa cada commit, identifica esta fraqueza exata e entrega um pull request pronto para revisão com o patch. Sem tickets. Sem transferências.

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Perguntas frequentes

Frequently asked questions

O que é o 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.

Qual a gravidade do CWE-135?

A MITRE não publicou uma classificação de probabilidade de exploração para esta fraqueza. Trate-a como impacto médio até o seu modelo de ameaças provar o contrário.

Que linguagens ou plataformas são afetadas pelo CWE-135?

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

Como posso prevenir o 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.)

Como é que o Plexicus deteta e corrige o CWE-135?

O motor SAST do Plexicus correlaciona a assinatura de fluxo de dados do CWE-135 em cada commit. Quando é encontrada uma correspondência, o nosso agente Codex Remedium abre um PR de correção com o código corrigido, testes e um resumo de uma linha para o revisor.

Onde posso saber mais sobre o CWE-135?

A MITRE publica a definição canónica em https://cwe.mitre.org/data/definitions/135.html. Pode também consultar a documentação da OWASP e do NIST para orientações adjacentes.

Fraquezas relacionadas

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Recursos

Further reading

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