CVE-2026-74963: Mozilla Firefox

Medium severity, CVSS 5.4. 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 154, Firefox ESR 140.14, Firefox ESR 153.1, Thunderbird 154, Thunderbird 140.14, and Thunderbird 153.1.

Affected products

• Mozilla Firefox: before 140.14.0 (fixed in 140.14.0); before 154.0.0 (fixed in 154.0.0)
• Mozilla Thunderbird: before 140.14.0 (fixed in 140.14.0); before 154.0 (fixed in 154.0); from 153.0, before 153.1.0 (fixed in 153.1.0)

Published 2026-08-18. Last modified 2026-08-19.