CVE-2025-5266: Mozilla Firefox

Medium severity, CVSS 4.3. EPSS: 0.3% chance of exploitation in the next 30 days.

Script elements loading cross-origin resources generated load and error events which leaked information enabling XS-Leaks attacks. 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)

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