CVE-2021-38503: Debian Linux

Critical severity, CVSS 10.0. EPSS: 4.1% chance of exploitation in the next 30 days.

The iframe sandbox rules were not correctly applied to XSLT stylesheets, allowing an iframe to bypass restrictions such as executing scripts or navigating the top-level frame. 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 (fixed in 91.3)
• Mozilla Thunderbird: before 91.3 (fixed in 91.3)

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