CVE-2026-100771: Mozilla Firefox

High severity, CVSS 8.1. EPSS: 0.3% chance of exploitation in the next 30 days.

Undefined behavior in the DOM: Streams component. This vulnerability was fixed in Firefox ESR 153.4, Thunderbird 157, Thunderbird 140.17, Thunderbird 153.4, Firefox 157, Firefox ESR 115.42, and Firefox ESR 140.17.

Affected products

• Mozilla Firefox: from 115.0, before 115.42.0 (fixed in 115.42.0); from 140.0.0, before 140.17.0 (fixed in 140.17.0); from 153.0, before 153.4.0 (fixed in 153.4.0); from 154.0.0, before 157.0.0 (fixed in 157.0.0)
• Mozilla Thunderbird: from 140.0, before 140.17.0 (fixed in 140.17.0); from 153.0, 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 2026-10-06.