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UnratedPublic exploit

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-controlled expressions into executable JavaScript using new Function(). Because crafted expressions are compiled directly into native JavaScript, an attacker can break out of the intended expression sandbox and execute arbitrary JavaScript within the application's runtime context. The issue is effectively an injection flaw in code generation logic exposed through expression compilation.

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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, compromise of application data, unauthorized actions using the application's privileges, integrity loss, service disruption, and potential further compromise of connected systems or secrets accessible to the process.

Mitigation

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

Do not pass untrusted or user-supplied expressions to toJSFunction(). Avoid direct compilation of user input into JavaScript. Restrict or disable features that convert expressions into executable code, and prefer safer evaluation approaches that do not rely on new Function() or equivalent dynamic code generation. Where immediate code changes are not possible, isolate the component, reduce process privileges, and limit access to sensitive resources to reduce blast radius.

Remediation

Patch, then assume compromise.

Update expr-eval to a vendor-fixed release that mitigates arbitrary JavaScript execution via toJSFunction() as soon as such a release is available. If no fixed version is currently available, remove or disable use of toJSFunction() wherever expressions may be influenced by untrusted input. Refactor affected code paths to avoid compiling expressions into executable JavaScript with new Function().
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.

EXPOSURE SURFACE

Affected products & vendors

Products and vendors Mallory has correlated with this vulnerability. Open in Mallory to drill down to specific CPE configurations and version ranges.

VendorProductType
SilentmattExpr-Evalapplication

Vendor-confirmed product mapping. Mallory continuously reconciles this list against your asset inventory.

ACTIVITY FEED

Recent activity

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

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

The version that knows your environment.

This page is what’s public. Mallory adds the parts that aren’t: which of your assets are affected, which adversaries are exploiting it right now, which detections to deploy, and what to do tonight.
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Exposure mapping

Query your assets running an affected version, and investigate the blast radius.

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 activity5

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