CVE-2021-4129: Mozilla Firefox

Critical severity, CVSS 9.8. EPSS: 1% chance of exploitation in the next 30 days.

Mozilla developers and community members Julian Hector, Randell Jesup, Gabriele Svelto, Tyson Smith, Christian Holler, and Masayuki Nakano reported memory safety bugs present in Firefox 94. Some of these bugs showed evidence of memory corruption and we presume that with enough effort some of these could have been exploited to run arbitrary code. This vulnerability affects Firefox < 95, Firefox ESR < 91.4.0, and Thunderbird < 91.4.0.

Affected products

• Mozilla Firefox: before 95.0 (fixed in 95.0)
• Mozilla Firefox ESR: before 91.4.0 (fixed in 91.4.0)
• Mozilla Thunderbird: before 91.4.0 (fixed in 91.4.0)

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