CVE-2023-6863: Debian Linux

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

The `ShutdownObserver()` was susceptible to potentially undefined behavior due to its reliance on a dynamic type that lacked a virtual destructor. This vulnerability affects Firefox ESR < 115.6, Thunderbird < 115.6, and Firefox < 121.

Affected products

• Debian Debian Linux: version 10.0 only; version 11.0 only; version 12.0 only
• Mozilla Firefox: before 121.0 (fixed in 121.0)
• Mozilla Firefox ESR: before 115.6 (fixed in 115.6)
• Mozilla Thunderbird: before 115.6 (fixed in 115.6)

Published 2023-12-19. Last modified 2026-06-17.