CWE-794 Variante Incompleto

Incomplete Filtering of Multiple Instances of Special Elements

This vulnerability occurs when an application receives data from a source but fails to remove or neutralize every instance of a potentially dangerous element before passing that data to another…

Definição

What is CWE-794?

This vulnerability occurs when an application receives data from a source but fails to remove or neutralize every instance of a potentially dangerous element before passing that data to another component.

This flaw is a common oversight in input validation and sanitization routines. It happens when a filter or sanitizer processes data but only handles the first, last, or a single instance of a special character, command, or code sequence (like '--' for SQL comments or '' tags). Attackers can bypass these incomplete defenses by injecting multiple instances of the element, allowing at least one to slip through and potentially cause injection, cross-site scripting (XSS), or other security issues. Incomplete filtering can affect both sequential elements (where dangerous patterns appear right next to each other) and non-sequential elements (where they are scattered throughout the input). To prevent this, developers must ensure their sanitization logic iterates through the entire input payload, removing or encoding all occurrences of the targeted special elements, not just one.

Impacto no mundo real

Real-world CVEs caused by CWE-794

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
The following code takes untrusted input and uses a regular expression to filter "../" from the input. It then appends this result to the /home/user/ directory and attempts to read the file in the final resulting path.
2
Since the regular expression does not have the /g global match modifier, it only removes the first instance of "../" it comes across. So an input value such as:
3
will have the first "../" stripped, resulting in:
4
This value is then concatenated with the /home/user/ directory:
5
which causes the /etc/passwd file to be retrieved once the operating system has resolved the ../ sequences in the pathname. This leads to relative path traversal (CWE-23).

Exemplo de código vulnerável

Vulnerable Perl

The following code takes untrusted input and uses a regular expression to filter "../" from the input. It then appends this result to the /home/user/ directory and attempts to read the file in the final resulting path.

Vulnerável Perl

my $Username = GetUntrustedInput();
  $Username =~ s/\.\.\///;
  my $filename = "/home/user/" . $Username;
  ReadAndSendFile($filename);

Payload do atacante

Since the regular expression does not have the /g global match modifier, it only removes the first instance of "../" it comes across. So an input value such as:

Payload do atacante

../../../etc/passwd

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-794

Architecture Use safe-by-default frameworks and APIs that prevent the unsafe pattern from being expressible.
Implementation Validate input at trust boundaries; use allowlists, not denylists.
Implementation Apply the principle of least privilege to credentials, file paths, and runtime permissions.
Testing Cover this weakness in CI: SAST rules + targeted unit tests for the data flow.
Operation Monitor logs for the runtime signals listed in the next section.

Sinais de deteção

How to detect CWE-794

SAST High

Executar análise estática (SAST) na base de código à procura do padrão inseguro no fluxo de dados.

DAST Moderate

Executar testes dinâmicos de segurança de aplicações (DAST) contra o endpoint em execução.

Runtime Moderate

Monitorizar os registos em tempo de execução para traços de exceção invulgares, input malformado ou tentativas de contornar a autorização.

Code review Moderate

Revisão de código: sinalizar qualquer novo código que trate input desta superfície sem usar os ajudantes validados do framework.

CWE-794

Don't catalog this weakness. Prove it's reachable.

Plexicus turns CWE catalogs into evidence: every CWE-pattern is matched against your real code graph, reach is proven on a sandbox clone, and verified findings ship as reviewed PRs.

Request AI Swarm Pentest

Perguntas frequentes

Frequently asked questions

O que é o CWE-794?

Qual a gravidade do CWE-794?

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-794?

A MITRE não especificou as plataformas afetadas por este CWE — pode aplicar-se à maioria das stacks de aplicações.

Como posso prevenir o CWE-794?

Use safe-by-default frameworks, validate untrusted input at trust boundaries, and apply the principle of least privilege. Cover the data-flow signature in CI with SAST.

Como é que o Plexicus deteta e corrige o CWE-794?

O motor SAST do Plexicus correlaciona a assinatura de fluxo de dados do CWE-794 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-794?

A MITRE publica a definição canónica em https://cwe.mitre.org/data/definitions/794.html. Pode também consultar a documentação da OWASP e do NIST para orientações adjacentes.

Fraquezas relacionadas

Weaknesses related to CWE-794

CWE-792 Pai

Incomplete Filtering of One or More Instances of Special Elements

This vulnerability occurs when an application receives external data but fails to properly neutralize all instances of potentially…

CWE-793 Irmão

Only Filtering One Instance of a Special Element

This vulnerability occurs when an application processes incoming data but only removes or neutralizes one occurrence of a dangerous…

Recursos

Pronto para validar o que importa.

O Plexicus é Proof-Driven AppSec: achados validados, compreensão contextual e remediação revisada — ancorada em evidência, escopada com você.

Reservar Pentest