CVE-2021-29951: Mozilla Firefox

Medium severity, CVSS 6.5. EPSS: 1.9% chance of exploitation in the next 30 days.

The Mozilla Maintenance Service granted SERVICE_START access to BUILTIN|Users which, in a domain network, grants normal remote users access to start or stop the service. This could be used to prevent the browser update service from operating (if an attacker spammed the 'Stop' command); but also exposed attack surface in the maintenance service. *Note: This issue only affected Windows operating systems older than Win 10 build 1709. Other operating systems are unaffected.*. This vulnerability affects Thunderbird < 78.10.1, Firefox < 87, and Firefox ESR < 78.10.1.

Affected products

• Mozilla Firefox: before 87.0 (fixed in 87.0)
• Mozilla Firefox ESR: before 78.10.1 (fixed in 78.10.1)
• Mozilla Thunderbird: before 78.10.1 (fixed in 78.10.1)

Published 2021-06-24. Last modified 2026-06-17.