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Arbitrary JavaScript Code Execution in expr-eval toJSFunction()

IdentifiersCVE-2026-12866CWE-94

CVE-2026-12866 is a critical code execution vulnerability affecting all versions of the expr-eval package. The flaw is in the toJSFunction() API, which transforms user-supplied expressions into executable JavaScript using the JavaScript new Function() constructor. Because attacker-controlled input is compiled directly into native code, a crafted expression can break out of the intended expression-evaluation sandbox and cause arbitrary JavaScript to execute within the hosting application's context.

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ANALYST BRIEF

Impact, mitigation & remediation

What it means. What to do now. Patch path, mitigations, and the assume-compromise checklist.

Impact

What an attacker gets, and what they’ve been doing with it.

Successful exploitation allows arbitrary JavaScript execution in the context of the vulnerable application. Depending on how expr-eval is used, this can lead to full remote code execution on the server-side runtime, unauthorized access to sensitive data available to the process, modification of application state, abuse of application privileges, and potential service disruption. The provided scoring indicates high confidentiality, integrity, and availability impact.

Mitigation

If you can’t patch tonight, do this now.

Do not pass untrusted or user-controlled expressions to toJSFunction(). Until a patched version is deployed, disable or avoid the toJSFunction() API for any attacker-influenced input, treat all expression input as untrusted, and isolate the application component using expr-eval to reduce blast radius. Where feasible, use safer parsing/evaluation approaches that do not compile input into executable JavaScript.

Remediation

Patch, then assume compromise.

Update expr-eval to a version that mitigates arbitrary JavaScript execution through the toJSFunction() API. If a fixed version is available from the maintainer, upgrade to that release immediately and review any code paths that compile user-controlled expressions. Replace unsafe dynamic compilation patterns where possible, especially any reliance on new Function() for untrusted input.
PUBLIC EXPLOITS

Exploits

No public exploits tracked yet. Mallory keeps watching.

VALID 0 / 0 TOTALView more in app

No public exploit code observed for this vulnerability.

ACTIVITY FEED

Recent activity

5 sources tracked across advisories, community write-ups, and news. New activity surfaces here as Mallory finds it.

5 SOURCESView more in app
What this page doesn’t show

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Exposure mapping

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Threat actor evidence

Every observed campaign linking this CVE to a named adversary.

Associated malware

Malware families riding this exploit, with evidence and IOCs.

Detection signatures

YARA, Sigma, Snort, and vendor rules, auto-deployed to your SIEM.

Vendor-by-vendor mapping

Cross-references every affected SKU, including bundled OEM variants.

Social activity4

Community discussion across Reddit, Mastodon, and other social sources.

EXTERNAL REFERENCES
MITRE CVE
cve.org · CVE-2026-12866
NVD
nvd.nist.gov/vuln/detail/CVE-2026-12866
MITRE CWE · CWE-94
cwe.mitre.org