CVE-2026-12866
Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.
NVD · uneditedAll versions of the package expr-eval are vulnerable to Code Execution via the toJSFunction() API. An attacker can execute arbitrary JavaScript by supplying crafted expressions that are compiled into native code using new Function(). Because user-controlled expressions are transformed directly into executable JavaScript, attackers can escape the intended expression sandbox and run arbitrary code within the application's context.
Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.
dbcve analysis · moderate confidenceThe expr-eval package allows code execution through its toJSFunction() API, which compiles user-supplied expressions into native JavaScript using new Function(). Attackers can escape the intended expression sandbox by crafting malicious expressions that execute arbitrary code within the application's context.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.
From the vector- Attack vector
- Network
- Complexity
- Low
- Privileges
- None
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- High
- Integrity
- High
- Availability
- High
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.
dbcve checksWork through these to decide whether this CVE applies to you.
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Identify expr-eval dependencySearch package.json, package-lock.json, or yarn.lock files for the 'expr-eval' library entryAffected if The expr-eval library is present in project dependencies
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Locate toJSFunction() usageSearch source code files for calls to 'toJSFunction' method, for example: toJSFunction( or .toJSFunction(Affected if Code contains any invocation of the toJSFunction() API
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Check input source to toJSFunctionTrace the data flow into the toJSFunction() argument. Examine if the expression parameter comes from user request bodies, URL parameters, API inputs, files, or database fields that may contain untrusted dataAffected if The expression argument to toJSFunction() originates from any user-controlled or external source including HTTP requests, file uploads, or untrusted storage
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Verify execution contextConfirm that the toJSFunction() call executes the compiled function, for example: toJSFunction(expr)() or similar patterns that actually invoke the returned functionAffected if The toJSFunction() output is actually called, executing the compiled JavaScript function
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Assess exposureDetermine if the application processes expressions from unauthenticated users, multiple tenants, or external APIs without validationAffected if The application accepts and processes expressions from untrusted or public sources without sandboxing
You are affected if your application uses the expr-eval library with toJSFunction() on expressions that originate from user-controlled or untrusted sources.
Generated from the published advisory. Verify against your own configuration.
Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.
From vendor dataAvoid using toJSFunction() with untrusted user input. Replace with safer expression evaluation alternatives that do not compile expressions to executable JavaScript code, or implement strict input validation and sandboxing if use of this API is unavoidable.
- Consultation4.0 h
- Implementation8.0 h
- Testing4.0 h
- Review / QA2.0 h
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Free · runs locallyCheck whether your project pulls in CVE-2026-12866 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2026-12866 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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- Verified mitigations, workarounds, and config changes
- Version or environment caveats, and links to real fixes
- No weaponised exploit code, or anything meant to cause harm
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