CVE-2021-4140: Mozilla Firefox

Critical severity, CVSS 10.0. EPSS: 1.4% chance of exploitation in the next 30 days.

It was possible to construct specific XSLT markup that would be able to bypass an iframe sandbox. This vulnerability affects Firefox ESR < 91.5, Firefox < 96, and Thunderbird < 91.5.

Affected products

• Mozilla Firefox: before 96.0 (fixed in 96.0)
• Mozilla Firefox ESR: before 91.5 (fixed in 91.5)
• Mozilla Thunderbird: before 91.5 (fixed in 91.5)

Published 2022-12-22. Last modified 2026-06-17.