CVE-2025-6433: Mozilla Firefox

Critical severity, CVSS 9.8. EPSS: 0.3% chance of exploitation in the next 30 days.

If a user visited a webpage with an invalid TLS certificate, and granted an exception, the webpage was able to provide a WebAuthn challenge that the user would be prompted to complete. This is in violation of the WebAuthN spec which requires "a secure transport established without errors". This vulnerability was fixed in Firefox 140 and Thunderbird 140.

Affected products

• Mozilla Firefox: before 140.0 (fixed in 140.0)

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