CVE-2026-74990: Mozilla Firefox

Critical severity, CVSS 9.8. EPSS: 0.7% chance of exploitation in the next 30 days.

Internally found bugs present in Thunderbird ESR 140.13, Thunderbird ESR 153.0 and Thunderbird 153. 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 154, Firefox ESR 115.39, Firefox ESR 140.14, Firefox ESR 153.1, Thunderbird 154, Thunderbird 140.14, and Thunderbird 153.1.

Affected products

• Mozilla Firefox: before 115.39.0 (fixed in 115.39.0); from 116.0, before 140.14.0 (fixed in 140.14.0); from 141.0, before 153.1.0 (fixed in 153.1.0)
• Mozilla Thunderbird: before 140.14.0 (fixed in 140.14.0); from 141.0, before 153.1.0 (fixed in 153.1.0)

Published 2026-08-18. Last modified 2026-09-01.