CVE-2025-5268: Mozilla Firefox

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

Memory safety bugs present in Firefox 138, Thunderbird 138, Firefox ESR 128.10, and Thunderbird 128.10. 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 139, Firefox ESR 128.11, Thunderbird 139, and Thunderbird 128.11.

Affected products

• Mozilla Firefox: before 128.11.0 (fixed in 128.11.0); before 139.0 (fixed in 139.0)
• Mozilla Thunderbird: before 128.11.0 (fixed in 128.11.0); from 129.0, before 139.0 (fixed in 139.0)

Published 2025-05-27. Last modified 2026-10-05.