CVE-2021-29976: Mozilla Firefox

High severity, CVSS 8.8. EPSS: 1.2% chance of exploitation in the next 30 days.

Mozilla developers reported memory safety bugs present in code shared between Firefox and Thunderbird. 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 Thunderbird < 78.12, Firefox ESR < 78.12, and Firefox < 90.

Affected products

• Mozilla Firefox: before 90.0 (fixed in 90.0)
• Mozilla Firefox ESR: before 78.12 (fixed in 78.12)
• Mozilla Thunderbird: before 78.12 (fixed in 78.12)

Published 2021-08-05. Last modified 2026-06-17.