Mozilla Firefox — CVE-2026-4691 (Critical)

Date Mar 24, 2026

Type Vendor advisory

Signal Critical vendor advisory

Vendor / Product Mozilla · Firefox

CVE CVE-2026-4691

Critical vendor advisory CVE-2026-4691

Summary

Use-after-free in the CSS Parsing and Computation component. This vulnerability was fixed in Firefox 149, Firefox ESR 115.34, Firefox ESR 140.9, Thunderbird 149, and Thunderbird 140.9.

Why it matters

This CVE carries a CRITICAL severity rating (CVSS 9.8) in Mozilla Firefox. Patch or mitigate promptly.

Operator check

Review CVE-2026-4691 in your asset inventory. Apply patches per vendor guidance and verify Firefox is not exposed. CVSS score: 9.8.

Sources

NVD — CVE-2026-4691 Vendor reference Vendor reference

PatchBrief uses public sources. It does not scan environments, verify exposure, or replace vendor guidance.

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