Code execution via unsafe toJSFunction compilation
Published Jun 23, 2026 · Updated Jun 23, 2026
Code injection in expr-eval allows attackers to execute arbitrary JavaScript in Node.js applications through toJSFunction(). Expression.prototype.toJSFunction() coerces supplied variable values with toString() and inserts the resulting text into source passed directly to new Function() without isolation. Exploitation requires an application to compile attacker-controlled variables with this API; successful code runs with the Node.js process's file and command privileges.
Summary
What happened
Code injection in expr-eval allows attackers to execute arbitrary JavaScript in Node.js applications through toJSFunction(). Expression.prototype.toJSFunction() coerces supplied variable values with toString() and inserts the resulting text into source passed directly to new Function() without isolation. Exploitation requires an application to compile attacker-controlled variables with this API; successful code runs with the Node.js process's file and command privileges.
The record
- CVE
- CVE-2026-12866
- Published
- Jun 23, 2026
- Updated
- Jun 23, 2026
- Vendor
- Matthew Crumley
- Product
- expr-eval
- Classifications
- CWE-94, T1059.007
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Jun 23, 2026CVE publishedPublication date reported by the CVE source.
- Jun 23, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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Is a vulnerable build present?
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- Affected versionversion=0 <*
What conditions does exploitation require?
What is affected?
Attacks
What attackers are doing with it
Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.
Weakness, pattern, technique
Public exploit references
- Node.js RCE via unsafe code generation in toJSFunction()proof of concept · demonstrated
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Technologies
Your stack
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Your stack
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