CVE-2026-74986: Mozilla Firefox

Critical severity, CVSS 9.1. EPSS: 0.6% chance of exploitation in the next 30 days.

Site isolation issue in the CSS Parsing and Computation component. This vulnerability was fixed in Firefox 154, Firefox ESR 153.1, Thunderbird 154, and Thunderbird 153.1.

Affected products

• Mozilla Firefox: before 153.1.0 (fixed in 153.1.0)
• Mozilla Thunderbird: before 153.1.0 (fixed in 153.1.0)

Published 2026-08-18. Last modified 2026-08-25.