CVE-2021-38505: Mozilla Firefox

Medium severity, CVSS 6.5. EPSS: 1.1% chance of exploitation in the next 30 days.

Microsoft introduced a new feature in Windows 10 known as Cloud Clipboard which, if enabled, will record data copied to the clipboard to the cloud, and make it available on other computers in certain scenarios. Applications that wish to prevent copied data from being recorded in Cloud History must use specific clipboard formats; and Firefox before versions 94 and ESR 91.3 did not implement them. This could have caused sensitive data to be recorded to a user's Microsoft account. *This bug only affects Firefox for Windows 10+ with Cloud Clipboard enabled. Other operating systems are unaffected.*. This vulnerability affects Firefox < 94, Thunderbird < 91.3, and Firefox ESR < 91.3.

Affected products

• Mozilla Firefox: before 94.0 (fixed in 94.0)
• Mozilla Firefox ESR: before 91.3.0 (fixed in 91.3.0)
• Mozilla Thunderbird: before 91.3.0 (fixed in 91.3.0)

Published 2021-12-08. Last modified 2026-06-17.