CVE-2025-0239: Mozilla Firefox

Medium severity, CVSS 4.0. EPSS: 0.2% chance of exploitation in the next 30 days.

When using Alt-Svc, ALPN did not properly validate certificates when the original server is redirecting to an insecure site. This vulnerability was fixed in Firefox 134, Firefox ESR 128.6, Thunderbird 134, and Thunderbird 128.6.

Affected products

• Mozilla Firefox: before 128.6.0 (fixed in 128.6.0); before 134.0 (fixed in 134.0)
• Mozilla Thunderbird: before 128.6.0 (fixed in 128.6.0); from 129.0, before 134.0 (fixed in 134.0)

Published 2025-01-07. Last modified 2026-10-05.