CVE-2026-84641: Mozilla Thunderbird

High severity, CVSS 7.5. EPSS: 0.3% chance of exploitation in the next 30 days.

A malicious IMAP server can trigger use-after-free and heap-memory disclosure by sending a crafted ID response. Heap contents can ultimately be persisted to prefs.js. This vulnerability was fixed in Thunderbird 155, Thunderbird 140.15, and Thunderbird 153.2.

Affected products

• Mozilla Thunderbird: before 140.15.0 (fixed in 140.15.0); from 141.0, before 153.2.0 (fixed in 153.2.0); from 154.0, before 155.0 (fixed in 155.0)

Published 2026-09-01. Last modified 2026-09-03.