CWE-917 Base Incomplete

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

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…

Definition

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.
Real-world impact

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)

How attackers exploit it

Step-by-step attacker path

  1. 1

    Identify a code path that handles untrusted input without validation.

  2. 2

    Craft a payload that exercises the unsafe behavior — injection, traversal, overflow, or logic abuse.

  3. 3

    Deliver the payload through a normal request and observe the application's reaction.

  4. 4

    Iterate until the response leaks data, executes attacker code, or escalates privileges.

Vulnerable code example

Vulnerable pseudo

MITRE has not published a code example for this CWE. The pattern below is illustrative — see Resources for canonical references.

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

Secure pseudo

Secure 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.
Prevention checklist

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".
Detection signals

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.)

Plexicus auto-fix

Plexicus auto-detects CWE-917 and opens a fix PR in under 60 seconds.

Codex Remedium scans every commit, identifies this exact weakness, and ships a reviewer-ready pull request with the patch. No tickets. No hand-offs.

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Frequently asked questions

Frequently asked questions

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.

How serious is CWE-917?

MITRE has not published a likelihood-of-exploit rating for this weakness. Treat it as medium-impact until your threat model proves otherwise.

What languages or platforms are affected by CWE-917?

MITRE lists the following affected platforms: Java.

How can I prevent 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

How does Plexicus detect and fix CWE-917?

Plexicus's SAST engine matches the data-flow signature for CWE-917 on every commit. When a match is found, our Codex Remedium agent opens a fix PR with the corrected code, tests, and a one-line summary for the reviewer.

Where can I learn more about CWE-917?

MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/917.html. You can also reference OWASP and NIST documentation for adjacent guidance.

Related weaknesses

Weaknesses related to CWE-917

CWE-77 Parent

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

This vulnerability occurs when an application builds a system command using untrusted user input without properly sanitizing it. An…

CWE-1427 Sibling

Improper Neutralization of Input Used for LLM Prompting

This vulnerability occurs when an application builds prompts for a Large Language Model (LLM) using external data, but does so in a way…

CWE-624 Sibling

Executable Regular Expression Error

This vulnerability occurs when an application uses a regular expression that can execute code, either because it directly contains…

CWE-78 Sibling

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

OS Command Injection occurs when an application builds a system command using untrusted, external input without properly sanitizing it.…

CWE-88 Sibling

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

This vulnerability occurs when an application builds a command string for execution by another component, but fails to properly separate…

CWE-1336 Peer

Improper Neutralization of Special Elements Used in a Template Engine

This vulnerability occurs when an application uses a template engine to process user-controlled input but fails to properly sanitize…

Resources

Further reading

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