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

Definição

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 no 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)

Como os atacantes a exploram

Trajeto do atacante passo a passo

1
Identificar um caminho de código que trata input não confiável sem validação.
2
Criar um payload que explora o comportamento inseguro — injeção, traversal, overflow ou abuso de lógica.
3
Entregar o payload através de um pedido normal e observar a reação da aplicação.
4
Iterar até que a resposta exponha dados, execute código do atacante ou escale privilégios.

Exemplo de código vulnerável

Vulnerable pseudo

A MITRE não publicou um exemplo de código para este CWE. O padrão abaixo é ilustrativo — consulte os Recursos para referências canónicas.

Vulnerável pseudo

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

Exemplo 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 verificação de prevenção

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".

Sinais de deteção

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

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Perguntas frequentes

Frequently asked questions

O que é o 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.

Qual a gravidade do CWE-917?

A MITRE não publicou uma classificação de probabilidade de exploração para esta fraqueza. Trate-a como impacto médio até o seu modelo de ameaças provar o contrário.

Que linguagens ou plataformas são afetadas pelo CWE-917?

MITRE lists the following affected platforms: Java.

Como posso prevenir o 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

Como é que o Plexicus deteta e corrige o CWE-917?

O motor SAST do Plexicus correlaciona a assinatura de fluxo de dados do CWE-917 em cada commit. Quando é encontrada uma correspondência, o nosso agente Codex Remedium abre um PR de correção com o código corrigido, testes e um resumo de uma linha para o revisor.

Onde posso saber mais sobre o CWE-917?

A MITRE publica a definição canónica em https://cwe.mitre.org/data/definitions/917.html. Pode também consultar a documentação da OWASP e do NIST para orientações adjacentes.

Fraquezas relacionadas

Weaknesses related to CWE-917

CWE-77 Pai

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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

Trajeto do atacante passo a passo

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`