CVE-2019-9791: Mozilla Firefox

Critical severity, CVSS 9.8. EPSS: 19.9% chance of exploitation in the next 30 days.

The type inference system allows the compilation of functions that can cause type confusions between arbitrary objects when compiled through the IonMonkey just-in-time (JIT) compiler and when the constructor function is entered through on-stack replacement (OSR). This allows for possible arbitrary reading and writing of objects during an exploitable crash. This vulnerability affects Thunderbird < 60.6, Firefox ESR < 60.6, and Firefox < 66.

Affected products

• Mozilla Firefox: before 60.6.0 (fixed in 60.6.0); before 66.0 (fixed in 66.0)
• Mozilla Thunderbird: before 60.6.0 (fixed in 60.6.0)
• Red Hat Enterprise Linux: version 8.0 only
• Red Hat Enterprise Linux Eus: version 8.1 only; version 8.2 only; version 8.4 only
• Red Hat Enterprise Linux Server Aus: version 8.2 only; version 8.4 only
• Red Hat Enterprise Linux Server Tus: version 8.2 only; version 8.4 only

Published 2019-04-26. Last modified 2026-06-17.