CVE-2026-0891: Mozilla Firefox

High severity, CVSS 8.1. EPSS: 0.5% chance of exploitation in the next 30 days.

Memory safety bugs present in Firefox ESR 140.6, Thunderbird ESR 140.6, Firefox 146 and Thunderbird 146. Some of these bugs showed evidence of memory corruption and we presume that with enough effort some of these could have been exploited to run arbitrary code. This vulnerability was fixed in Firefox 147, Firefox ESR 140.7, Thunderbird 147, and Thunderbird 140.7.

Affected products

• Mozilla Firefox: before 140.7.0 (fixed in 140.7.0); before 147.0 (fixed in 147.0)
• Mozilla Thunderbird: before 140.7.0 (fixed in 140.7.0); before 147.0 (fixed in 147.0)

Published 2026-01-13. Last modified 2026-07-15.