CVE-2026-12304: Mozilla Firefox

Critical severity, CVSS 9.1. EPSS: 0.2% chance of exploitation in the next 30 days.

Same-origin policy bypass in the Networking: Cookies component. This vulnerability was fixed in Firefox 152, Firefox ESR 140.12, Thunderbird 152, and Thunderbird 140.12.

Affected products

• Mozilla Firefox: before 140.12.0 (fixed in 140.12.0); before 152.0.0 (fixed in 152.0.0)
• Mozilla Thunderbird: before 140.12.0 (fixed in 140.12.0); before 152.0.0 (fixed in 152.0.0)

Published 2026-06-16. Last modified 2026-06-17.