CVE-2021-29987: Mozilla Firefox

Medium severity, CVSS 6.5. EPSS: 0.9% chance of exploitation in the next 30 days.

After requesting multiple permissions, and closing the first permission panel, subsequent permission panels will be displayed in a different position but still record a click in the default location, making it possible to trick a user into accepting a permission they did not want to. *This bug only affects Firefox on Linux. Other operating systems are unaffected.*. This vulnerability affects Firefox < 91 and Thunderbird < 91.

Affected products

• Mozilla Firefox: before 91.0 (fixed in 91.0)
• Mozilla Thunderbird: before 91.0 (fixed in 91.0)

Published 2021-08-17. Last modified 2026-06-17.