CVE-2021-29981: Mozilla Firefox

High severity, CVSS 8.8. EPSS: 1% chance of exploitation in the next 30 days.

An issue present in lowering/register allocation could have led to obscure but deterministic register confusion failures in JITted code that would lead to a potentially exploitable crash. This vulnerability affects Firefox < 91 and Thunderbird < 91.

Affected products

• Mozilla Firefox: before 91.0 (fixed in 91.0)
• Mozilla Thunderbird: before 91.0 (fixed in 91.0)

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