CVE-2021-38509: Debian Linux

Medium severity, CVSS 4.3. EPSS: 1.8% chance of exploitation in the next 30 days.

Due to an unusual sequence of attacker-controlled events, a Javascript alert() dialog with arbitrary (although unstyled) contents could be displayed over top an uncontrolled webpage of the attacker's choosing. 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.