CVE-2025-6429: Mozilla Firefox

Medium severity, CVSS 6.5. EPSS: 0.4% chance of exploitation in the next 30 days.

Firefox could have incorrectly parsed a URL and rewritten it to the youtube.com domain when parsing the URL specified in an `embed` tag. This could have bypassed website security checks that restricted which domains users were allowed to embed. This vulnerability was fixed in Firefox 140, Firefox ESR 128.12, Thunderbird 140, and Thunderbird 128.12.

Affected products

• Mozilla Firefox: before 128.12.0 (fixed in 128.12.0); before 140.0 (fixed in 140.0)

Published 2025-06-24. Last modified 2026-09-30.