CVE-2026-84124: Mozilla Firefox

Medium severity, CVSS 5.4. EPSS: 0.2% chance of exploitation in the next 30 days.

Use-after-free in the DOM: Core & HTML component. This vulnerability was fixed in Firefox 155, Firefox ESR 140.15, Firefox ESR 153.2, Thunderbird 155, Thunderbird 140.15, and Thunderbird 153.2.

Affected products

• Mozilla Firefox: before 140.15.0 (fixed in 140.15.0); from 141.0.0, before 153.2.0 (fixed in 153.2.0); from 154.0.0, before 155.0.0 (fixed in 155.0.0)
• Mozilla Thunderbird: before 140.15.0 (fixed in 140.15.0); from 141.0, before 153.2.0 (fixed in 153.2.0); from 154.0, before 155.0 (fixed in 155.0)

Published 2026-09-01. Last modified 2026-09-02.