CVE-2026-100816: Mozilla Firefox

High severity, CVSS 8.1. EPSS: 0.2% chance of exploitation in the next 30 days.

Site isolation issue in the DOM: Networking component. This vulnerability was fixed in Firefox ESR 153.4, Thunderbird 157, Thunderbird 153.4, and Firefox 157.

Affected products

• Mozilla Firefox: before 153.4.0 (fixed in 153.4.0); from 154.0.0, before 157.0.0 (fixed in 157.0.0)
• Mozilla Thunderbird: before 153.4.0 (fixed in 153.4.0); from 154.0, before 157.0 (fixed in 157.0)

Published 2026-09-29. Last modified 2026-10-05.