CWE-790: Improper Filtering of Special Elements

Definición

What is CWE-790?

This vulnerability occurs when an application accepts data from a source but fails to properly sanitize or incorrectly filters out special characters or control elements before passing that data to another system component.

At its core, this weakness is about a broken chain of trust in data handling. An application often assumes data from an upstream component (like a user, another service, or a database) is safe, but it must actively validate and neutralize potentially dangerous elements—such as quotes, brackets, script tags, or command delimiters—before that data flows downstream. If this filtering step is missing or flawed, the downstream component interprets these special elements as part of its own commands or code, leading to security breaches. For developers, this means you cannot rely on the source of your data. You must implement context-aware filtering or encoding at the point where data is used. For example, data destined for SQL needs parameterized queries, data for HTML output needs HTML entity encoding, and data for system commands needs strict allow-list validation. The fix isn't a single filter; it's applying the correct defense for each specific output context to prevent injections, cross-site scripting (XSS), and command execution attacks.

Impacto en el mundo real

Real-world CVEs caused by CWE-790

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
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
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
will have the first "../" stripped, resulting in:
4
This value is then concatenated with the /home/user/ directory:
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-790

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-790

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.

CWE-790

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.

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Preguntas frecuentes

Frequently asked questions

¿Qué es CWE-790?

This vulnerability occurs when an application accepts data from a source but fails to properly sanitize or incorrectly filters out special characters or control elements before passing that data to another system component.

¿Qué gravedad tiene CWE-790?

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-790?

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

¿Cómo puedo prevenir CWE-790?

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-790?

El motor SAST de Plexicus detecta la firma de flujo de datos para CWE-790 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-790?

MITRE publica la definición canónica en https://cwe.mitre.org/data/definitions/790.html. También puedes consultar la documentación de OWASP y NIST para guías relacionadas.

Debilidades relacionadas

Weaknesses related to CWE-790

CWE-138 Padre

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Reservar Pentest

Improper Filtering of Special Elements

What is CWE-790?

Real-world CVEs caused by CWE-790

Ruta del atacante paso a paso

Vulnerable Perl

Secure pseudo

How to prevent CWE-790

How to detect CWE-790

Don't catalog this weakness. Prove it's reachable.

Frequently asked questions

Weaknesses related to CWE-790

Improper Neutralization of Special Elements

Improper Neutralization of Delimiters

Improper Neutralization of Input Terminators

Improper Neutralization of Input Leaders

Improper Neutralization of Quoting Syntax

Improper Neutralization of Escape, Meta, or Control Sequences

Improper Neutralization of Comment Delimiters

Improper Neutralization of Macro Symbols

Improper Neutralization of Substitution Characters

Further reading

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Sample evidence handover

Server-Side Request Forgery in `webhooks/receiver`