CVE-2021-38506: Debian Linux

Medium severity, CVSS 4.3. EPSS: 1.5% chance of exploitation in the next 30 days.

Through a series of navigations, Firefox could have entered fullscreen mode without notification or warning to the user. This could lead to spoofing attacks on the browser UI including phishing. This vulnerability affects Firefox < 94, Thunderbird < 91.3, and Firefox ESR < 91.3.

Affected products

• Debian Debian Linux: version 9.0 only; version 10.0 only; version 11.0 only
• 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.