CWE-796 Variante Incompleto

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 (like the start or end of a string). It fails to detect and…

Definición

What is CWE-796?

This vulnerability occurs when software filters dangerous inputs or characters, but only checks for them in specific, expected locations (like the start or end of a string). It fails to detect and remove the same dangerous elements if they appear elsewhere in the data, allowing them to pass through to critical system components.
Imagine a security filter designed to strip out SQL comment sequences (like '--') only if they appear at the very beginning of a user-provided string. If an attacker injects that same sequence in the middle of the input (e.g., `admin'--`), the filter misses it entirely. The downstream database, receiving the unfiltered input, then interprets the sequence as legitimate SQL, potentially leading to unauthorized access or data manipulation. This flaw stems from an incomplete validation logic that makes a dangerous assumption: that malicious payloads only appear in predictable spots. Effective sanitization must scan and clean the *entire* input, regardless of position. Developers should use context-aware encoding or parameterized interfaces instead of relying on positional filtering, as attackers will always probe for and exploit these blind spots.
Impacto en el mundo real

Real-world CVEs caused by CWE-796

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 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).

Ejemplo de código vulnerable

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.

Vulnerable Perl 
my $Username = GetUntrustedInput();
  $Username =~ s/^\.\.\///;
  my $filename = "/home/user/" . $Username;
  ReadAndSendFile($filename);
Payload del 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 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-796

  • 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-796

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-796 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-796?

This vulnerability occurs when software filters dangerous inputs or characters, but only checks for them in specific, expected locations (like the start or end of a string). It fails to detect and remove the same dangerous elements if they appear elsewhere in the data, allowing them to pass through to critical system components.

¿Qué gravedad tiene CWE-796?

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-796?

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

¿Cómo puedo prevenir CWE-796?

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-796?

El motor SAST de Plexicus detecta la firma de flujo de datos para CWE-796 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-796?

MITRE publica la definición canónica en https://cwe.mitre.org/data/definitions/796.html. También puedes consultar la documentación de OWASP y NIST para guías relacionadas.

Debilidades relacionadas

Weaknesses related to CWE-796

CWE-795 Padre

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…

CWE-797 Hermano

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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