CVE-2021-23964: Mozilla Firefox

High severity, CVSS 8.8. EPSS: 1.4% chance of exploitation in the next 30 days.

Mozilla developers reported memory safety bugs present in Firefox 84 and Firefox ESR 78.6. 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 < 85, Thunderbird < 78.7, and Firefox ESR < 78.7.

Affected products

• Mozilla Firefox: before 85.0 (fixed in 85.0)
• Mozilla Firefox ESR: before 78.7 (fixed in 78.7)
• Mozilla Thunderbird: before 78.7 (fixed in 78.7)

Published 2021-02-26. Last modified 2026-06-17.