A widely used JS expression evaluator vulnerability enabling arbitrary code execution in apps that incorporate expr-eval.
CVE-2026-12866
Code execution via toJSFunction in expr-eval permits arbitrary JS.
Is CVE-2026-12866 being exploited?
Not confirmed. CVE-2026-12866 does not appear in CISA's Known Exploited Vulnerabilities catalog, which records only exploitation that has been observed and reported publicly. That is evidence of absence of a report, not evidence the flaw is unattacked. EPSS currently estimates a 0.87% probability of exploitation in the next 30 days.
How severe is CVE-2026-12866?
CVE-2026-12866 is rated Critical with a CVSS score of 9.2. Severity describes how bad exploitation would be, not how likely it is: pair it with exploitation evidence before deciding what to patch first.
Is there a patch for CVE-2026-12866?
Not known from our data. No patch reference has been recorded for CVE-2026-12866, which is not the same as no patch existing — a fix may have shipped without a tagged reference, or after this record was written. The vendor advisory is the only authority on whether a fix exists, and mitigations may be available regardless.
What does CVE-2026-12866 affect?
CVE-2026-12866 affects expr-eval library. Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
What should I do about CVE-2026-12866?
Upgrade expr-eval to latest secure version; audit toJSFunction usage.
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
Arbitrary code execution in apps using expr-eval; crafted expressions run with app privileges.
Immediate action required
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- affected< *
As published in the CVE Program record. A version outside these ranges is not a statement that it is unaffected — vendors sometimes understate a range, and distribution-backported builds carry upstream numbers that do not reflect what was patched into them.