CWE-795 Base Incompleto

Only Filtering Special Elements at a Specified Location

This vulnerability occurs when a security filter only checks for dangerous input patterns at specific, predefined locations within the data. It fails to scan the entire input stream, allowing…

Definição

What is CWE-795?

This vulnerability occurs when a security filter only checks for dangerous input patterns at specific, predefined locations within the data. It fails to scan the entire input stream, allowing malicious elements that appear outside the expected location to pass through and potentially harm downstream components.
Imagine a filter designed to remove semicolons, but it only checks the very beginning of a user-supplied string. An attacker can simply place the semicolon later in the input—for example, in the middle or at the end—to bypass the filter entirely. This creates a false sense of security because the filter is active, but its limited scope leaves the application exposed to the very attacks it was meant to prevent. This often happens when validation logic uses functions that only look for special characters at absolute positions (like 'byte 10') or relative to markers (like 'the second argument'). Developers must ensure their input sanitization processes perform a comprehensive check across the entire data payload, not just a single, predictable spot. The core failure is a logic error in the filter's design, not the absence of a filter.
Impacto no mundo real

Real-world CVEs caused by CWE-795

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

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

  2. 2

    Since the regular expression is only looking for an instance of "../" at the beginning of the string, it only removes the first "../" element. So an input value such as:

  3. 3

    will have the first "../" stripped, resulting in:

  4. 4

    This value is then concatenated with the /home/user/ directory:

  5. 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-22).

Exemplo de código vulnerável

Vulnerable Perl

The following code takes untrusted input and uses a regular expression to filter a "../" element located at the beginning of the input string. 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 is only looking for an instance of "../" at the beginning of the string, it only removes the first "../" element. 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-795

  • 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-795

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.

Correção automática do Plexicus

O Plexicus deteta automaticamente o CWE-795 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-795?

This vulnerability occurs when a security filter only checks for dangerous input patterns at specific, predefined locations within the data. It fails to scan the entire input stream, allowing malicious elements that appear outside the expected location to pass through and potentially harm downstream components.

Qual a gravidade do CWE-795?

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-795?

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

Como posso prevenir o CWE-795?

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-795?

O motor SAST do Plexicus correlaciona a assinatura de fluxo de dados do CWE-795 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-795?

A MITRE publica a definição canónica em https://cwe.mitre.org/data/definitions/795.html. Pode também consultar a documentação da OWASP e do NIST para orientações adjacentes.

Fraquezas relacionadas

Weaknesses related to CWE-795

CWE-791 Pai

Incomplete Filtering of Special Elements

This vulnerability occurs when an application accepts data from a source but fails to properly clean or neutralize all special characters…

CWE-792 Irmão

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-796 Filho

Only Filtering Special Elements Relative to a Marker

This vulnerability occurs when software filters dangerous inputs or characters, but only checks for them in specific, expected locations…

CWE-797 Filho

Only Filtering Special Elements at an Absolute Position

This vulnerability occurs when software checks for dangerous characters or patterns only at a fixed, hardcoded location in input data.…

Recursos

Further reading

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