CVE-2026-4691: Mozilla Firefox

Critical severity, CVSS 9.8. EPSS: 0.5% chance of exploitation in the next 30 days.

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.

Affected products

• Mozilla Firefox: before 115.34.0 (fixed in 115.34.0); before 149.0 (fixed in 149.0); from 128.0, before 140.9.0 (fixed in 140.9.0)

Published 2026-03-24. Last modified 2026-07-15.