Executar análise estática (SAST) na base de código à procura do padrão inseguro no fluxo de dados.
CWE-177 Variante Rascunho
Improper Handling of URL Encoding (Hex Encoding)
This vulnerability occurs when an application fails to correctly process URL-encoded (also known as percent-encoded or hex-encoded) input, either by double-decoding it, not decoding it at all, or…
Definição
What is CWE-177?
This vulnerability occurs when an application fails to correctly process URL-encoded (also known as percent-encoded or hex-encoded) input, either by double-decoding it, not decoding it at all, or inconsistently handling encoded characters across different security checks.
URL encoding (like %20 for a space) is a standard way to safely transmit special characters in web requests. The security flaw arises when an application's validation logic, filtering routines, and business logic treat the same encoded input differently. For example, a security filter might check the raw encoded string, while a later processing step decodes it, allowing malicious payloads to slip through undetected.
This inconsistency creates a classic security bypass. An attacker can embed malicious code (like SQL commands or script tags) within encoded sequences that evade initial validation. To prevent this, developers must ensure all security controls—input validation, output encoding, and business logic—operate on the same canonical, decoded representation of the data, and apply decoding only once, at the very beginning of the request pipeline.
Impacto no mundo real
Real-world CVEs caused by CWE-177
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Hex-encoded path traversal variants - "%2e%2e", "%2e%2e%2f", "%5c%2e%2e"
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Hex-encoded path traversal variants - "%2e%2e", "%2e%2e%2f", "%5c%2e%2e"
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Hex-encoded path traversal variants - "%2e%2e", "%2e%2e%2f", "%5c%2e%2e"
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"%20" (encoded space)
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"%20" (encoded space)
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"%20" (encoded space)
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"%20" (encoded space)
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Crash via hex-encoded space "%20".
Como os atacantes a exploram
Trajeto do atacante passo a passo
- 1
Identificar um caminho de código que trata input não confiável sem validação.
- 2
Criar um payload que explora o comportamento inseguro — injeção, traversal, overflow ou abuso de lógica.
- 3
Entregar o payload através de um pedido normal e observar a reação da aplicação.
- 4
Iterar até que a resposta exponha dados, execute código do atacante ou escale privilégios.
Exemplo de código vulnerável
Vulnerable pseudo
A MITRE não publicou um exemplo de código para este CWE. O padrão abaixo é ilustrativo — consulte os Recursos para referências canónicas.
// Example pattern — see MITRE for the canonical references.
function handleRequest(input) {
// Untrusted input flows directly into the sensitive sink.
return executeUnsafe(input);
} Exemplo de código seguro
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.
Lista de verificação de prevenção
How to prevent CWE-177
- Architecture and Design Avoid making decisions based on names of resources (e.g. files) if those resources can have alternate names.
- Implementation Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does. When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, "boat" may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as "red" or "blue." Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code's environment changes. This can give attackers enough room to bypass the intended validation. However, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.
- Implementation Inputs should be decoded and canonicalized to the application's current internal representation before being validated (CWE-180). Make sure that the application does not decode the same input twice (CWE-174). Such errors could be used to bypass allowlist validation schemes by introducing dangerous inputs after they have been checked.
Sinais de deteção
How to detect CWE-177
Executar testes dinâmicos de segurança de aplicações (DAST) contra o endpoint em execução.
Monitorizar os registos em tempo de execução para traços de exceção invulgares, input malformado ou tentativas de contornar a autorização.
Revisão de código: sinalizar qualquer novo código que trate input desta superfície sem usar os ajudantes validados do framework.
CWE-177
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.
Perguntas frequentes O que é o CWE-177?
Qual a gravidade do CWE-177?
Que linguagens ou plataformas são afetadas pelo CWE-177?
Como posso prevenir o CWE-177?
Como é que o Plexicus deteta e corrige o CWE-177?
Onde posso saber mais sobre o CWE-177?
Frequently asked questions
O que é o CWE-177?
This vulnerability occurs when an application fails to correctly process URL-encoded (also known as percent-encoded or hex-encoded) input, either by double-decoding it, not decoding it at all, or inconsistently handling encoded characters across different security checks.
Qual a gravidade do CWE-177?
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-177?
A MITRE não especificou as plataformas afetadas por este CWE — pode aplicar-se à maioria das stacks de aplicações.
Como posso prevenir o CWE-177?
Avoid making decisions based on names of resources (e.g. files) if those resources can have alternate names. Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does. When performing input validation, consider all potentially relevant properties, including length, type of input, the…
Como é que o Plexicus deteta e corrige o CWE-177?
O motor SAST do Plexicus correlaciona a assinatura de fluxo de dados do CWE-177 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-177?
A MITRE publica a definição canónica em https://cwe.mitre.org/data/definitions/177.html. Pode também consultar a documentação da OWASP e do NIST para orientações adjacentes.
Fraquezas relacionadas
Weaknesses related to CWE-177
CWE-172 Pai
Encoding Error
This vulnerability occurs when software incorrectly transforms data between different formats, leading to corrupted or misinterpreted…
CWE-173 Irmão
Improper Handling of Alternate Encoding
This vulnerability occurs when software fails to correctly process input that arrives in a different character encoding than expected,…
CWE-174 Irmão
Double Decoding of the Same Data
This vulnerability occurs when an application decodes the same piece of data twice in sequence. This double processing can bypass or…
CWE-175 Irmão
Improper Handling of Mixed Encoding
This vulnerability occurs when software fails to correctly process input that contains multiple character encodings within the same data…
CWE-176 Irmão
Improper Handling of Unicode Encoding
This vulnerability occurs when software fails to correctly process or interpret Unicode-encoded input, leading to security bypasses, data…
Pronto para validar o que importa?
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.