CVE-2026-100803: Mozilla Firefox

High severity, CVSS 8.1. EPSS: 0.2% chance of exploitation in the next 30 days.

Same-origin policy bypass in the WebExtensions component. This vulnerability was fixed in Firefox ESR 153.4, Thunderbird 157, Thunderbird 140.17, Thunderbird 153.4, Firefox 157, Firefox ESR 115.42, and Firefox ESR 140.17.

Affected products

• Mozilla Firefox
• Mozilla Thunderbird

Published 2026-09-29. Last modified 2026-10-01.