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…

Definición

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 en el mundo real

Real-world CVEs caused by CWE-794

Todavía no hay CVEs públicos enlazados a esta CWE en el catálogo de MITRE.

Cómo lo explotan los atacantes

Ruta del atacante paso a paso

  1. 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. 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. 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-23).

Ejemplo de código vulnerable

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.

Vulnerable Perl 
my $Username = GetUntrustedInput();
  $Username =~ s/\.\.\///;
  my $filename = "/home/user/" . $Username;
  ReadAndSendFile($filename);
Payload del 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 del atacante 
../../../etc/passwd
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-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.
Señales de detección

How to detect CWE-794

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-794 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-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.

¿Qué gravedad tiene CWE-794?

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-794?

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

¿Cómo puedo prevenir 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.

¿Cómo detecta y corrige Plexicus CWE-794?

El motor SAST de Plexicus detecta la firma de flujo de datos para CWE-794 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-794?

MITRE publica la definición canónica en https://cwe.mitre.org/data/definitions/794.html. También puedes consultar la documentación de OWASP y NIST para guías relacionadas.

Debilidades relacionadas

Weaknesses related to CWE-794

CWE-792 Padre

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 Hermano

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

Further reading

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