CVE-2026-84145: Mozilla Firefox

High severity, CVSS 7.5. EPSS: 0.4% chance of exploitation in the next 30 days.

Internally found bugs present in Thunderbird 154, Thunderbird ESR 153.1 and Thunderbird ESR 140.14. Some of these bugs showed evidence of memory corruption or another security-relevant defect and we presume that with enough effort some of these could have been exploited. This vulnerability was fixed in Firefox 155, Firefox ESR 115.40, Firefox ESR 140.15, Firefox ESR 153.2, Thunderbird 155, Thunderbird 140.15, and Thunderbird 153.2.

Affected products

• Mozilla Firefox: before 115.40.0 (fixed in 115.40.0); from 116.0, 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 2026-09-03.