CVE-2021-23960: Mozilla Firefox

High severity, CVSS 8.8. EPSS: 1.3% chance of exploitation in the next 30 days.

Performing garbage collection on re-declared JavaScript variables resulted in a user-after-poison, and a potentially exploitable crash. 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.