CWE-177 Variante Borrador

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…

Definición

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 en el mundo real

Real-world CVEs caused by CWE-177

Cómo lo explotan los atacantes

Ruta del atacante paso a paso

  1. 1

    Identifica una ruta de código que maneje entrada no confiable sin validación.

  2. 2

    Crea un payload que ejercite el comportamiento inseguro — inyección, traversal, overflow o abuso de lógica.

  3. 3

    Envía el payload a través de una solicitud normal y observa la reacción de la aplicación.

  4. 4

    Itera hasta que la respuesta filtre datos, ejecute código del atacante o escale privilegios.

Ejemplo de código vulnerable

Vulnerable pseudo

MITRE no ha publicado un ejemplo de código para esta CWE. El patrón siguiente es ilustrativo — consulta Recursos para referencias canónicas.

Vulnerable pseudo 
// Example pattern — see MITRE for the canonical references.
function handleRequest(input) {
  // Untrusted input flows directly into the sensitive sink.
  return executeUnsafe(input);
}
Ejemplo 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 prevención

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.
Señales de detección

How to detect CWE-177

SAST High

Ejecuta análisis estático (SAST) sobre el código buscando el patrón inseguro en el flujo de datos.

DAST Moderate

Ejecuta pruebas dinámicas de seguridad de aplicaciones (DAST) contra el endpoint en vivo.

Runtime Moderate

Vigila los logs en tiempo de ejecución para detectar trazas de excepción inusuales, entradas malformadas o intentos de bypass de autorización.

Code review Moderate

Revisión de código: marca cualquier código nuevo que maneje entrada desde esta superficie sin usar los helpers validados del framework.

Auto-corrección de Plexicus

Plexicus detecta automáticamente CWE-177 y abre un PR de corrección en menos de 60 segundos.

Codex Remedium escanea cada commit, identifica esta debilidad concreta y entrega un pull request listo para revisión con el parche. Sin tickets. Sin traspasos.

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Preguntas frecuentes

Frequently asked questions

¿Qué es 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.

¿Qué gravedad tiene CWE-177?

MITRE no ha publicado una calificación de probabilidad de explotación para esta debilidad. Trátala como de impacto medio hasta que tu modelo de amenazas demuestre lo contrario.

¿Qué lenguajes o plataformas se ven afectados por CWE-177?

MITRE no ha especificado plataformas afectadas para esta CWE — puede aplicar a la mayoría de los stacks de aplicaciones.

¿Cómo puedo prevenir 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…

¿Cómo detecta y corrige Plexicus CWE-177?

El motor SAST de Plexicus detecta la firma de flujo de datos para CWE-177 en cada commit. Cuando hay coincidencia, nuestro agente Codex Remedium abre un PR de corrección con el código corregido, las pruebas y un resumen de una línea para el revisor.

¿Dónde puedo aprender más sobre CWE-177?

MITRE publica la definición canónica en https://cwe.mitre.org/data/definitions/177.html. También puedes consultar la documentación de OWASP y NIST para guías relacionadas.

Debilidades relacionadas

Weaknesses related to CWE-177

CWE-172 Padre

Encoding Error

This vulnerability occurs when software incorrectly transforms data between different formats, leading to corrupted or misinterpreted…

CWE-173 Hermano

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 Hermano

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 Hermano

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 Hermano

Improper Handling of Unicode Encoding

This vulnerability occurs when software fails to correctly process or interpret Unicode-encoded input, leading to security bypasses, data…

Recursos

Further reading

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