CWE-917: Improper Neutralization of Special Elements used in an Expression Language Statement ('Expression Language Injection')

Definición

What is CWE-917?

Expression Language Injection occurs when an application uses untrusted, external input to build an expression language statement—common in frameworks like Java Server Pages (JSP)—without properly sanitizing it. This allows an attacker to inject malicious expressions that alter the intended logic and execute arbitrary code when the statement is processed.

Frameworks like JSP allow developers to embed dynamic expressions directly into web pages. If these expression evaluation features are left enabled and user input flows into them without validation, attackers can craft inputs that break out of the intended data context. The injected expressions are then executed on the server, potentially leading to data exposure, system compromise, or other unexpected behavior. Preventing this requires disabling expression evaluation where it's not needed, rigorously validating and sanitizing all user inputs that touch expression contexts, and adopting a secure coding policy for the framework. Managing this at scale across a large codebase is difficult; an ASPM like Plexicus can help you track and remediate these flaws across your entire stack. While SAST tools catch the vulnerable pattern, Plexicus uses AI to suggest the actual code fix, saving hours of manual security work.

Impacto en el mundo real

Real-world CVEs caused by CWE-917

CVE-2021-44228

Product does not neutralize ${xyz} style expressions, allowing remote code execution. (log4shell vulnerability in log4j)

Cómo lo explotan los atacantes

Ruta del atacante paso a paso

1
Identifica una ruta de código que maneje entrada no confiable sin validación.
2
Crea un payload que ejercite el comportamiento inseguro — inyección, traversal, overflow o abuso de lógica.
3
Envía el payload a través de una solicitud normal y observa la reacción de la aplicación.
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-917

Architecture and Design Avoid adding user-controlled data into an expression interpreter when possible.
Implementation If user-controlled data must be added to an expression interpreter, one or more of the following should be performed: - Validate that the user input will not evaluate as an expression - Encode the user input in a way that ensures it is not evaluated as an expression
System Configuration / Operation The framework or tooling might allow the developer to disable or deactivate the processing of EL expressions, such as setting the isELIgnored attribute for a JSP page to "true".

Señales de detección

How to detect CWE-917

Automated Static Analysis High

Automated static analysis, commonly referred to as Static Application Security Testing (SAST), can find some instances of this weakness by analyzing source code (or binary/compiled code) without having to execute it. Typically, this is done by building a model of data flow and control flow, then searching for potentially-vulnerable patterns that connect "sources" (origins of input) with "sinks" (destinations where the data interacts with external components, a lower layer such as the OS, etc.)

CWE-917

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-917?

Expression Language Injection occurs when an application uses untrusted, external input to build an expression language statement—common in frameworks like Java Server Pages (JSP)—without properly sanitizing it. This allows an attacker to inject malicious expressions that alter the intended logic and execute arbitrary code when the statement is processed.

¿Qué gravedad tiene CWE-917?

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-917?

MITRE lists the following affected platforms: Java.

¿Cómo puedo prevenir CWE-917?

Avoid adding user-controlled data into an expression interpreter when possible. If user-controlled data must be added to an expression interpreter, one or more of the following should be performed: - Validate that the user input will not evaluate as an expression - Encode the user input in a way that ensures it is not evaluated as an expression

¿Cómo detecta y corrige Plexicus CWE-917?

El motor SAST de Plexicus detecta la firma de flujo de datos para CWE-917 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-917?

MITRE publica la definición canónica en https://cwe.mitre.org/data/definitions/917.html. También puedes consultar la documentación de OWASP y NIST para guías relacionadas.

Debilidades relacionadas

Weaknesses related to CWE-917

CWE-77 Padre

Listo para validar lo que importa.

Plexicus es Proof-Driven AppSec: hallazgos validados, comprensión contextual y remediación revisada — anclada en evidencia, acotada contigo.

Reservar Pentest

Improper Neutralization of Special Elements used in an Expression Language Statement ('Expression Language Injection')

What is CWE-917?

Real-world CVEs caused by CWE-917

Ruta del atacante paso a paso

Vulnerable pseudo

Secure pseudo

How to prevent CWE-917

How to detect CWE-917

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

Frequently asked questions

Weaknesses related to CWE-917

Improper Neutralization of Special Elements used in a Command ('Command Injection')

Improper Neutralization of Input Used for LLM Prompting

Executable Regular Expression Error

Improper Neutralization of Special Elements used in an OS Command ('OS Command Injection')

Improper Neutralization of Argument Delimiters in a Command ('Argument Injection')

Improper Neutralization of Special Elements Used in a Template Engine

Further reading

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Sample evidence handover

Server-Side Request Forgery in `webhooks/receiver`