CVE-2026-84641: Information disclosure due to malicious IMAP server response

Severity: high (CVSS 7.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.3% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-09-01 · updated 2026-09-03
Weakness: CWE-416

Description

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; from 141.0 before 153.2.0; from 154.0 before 155.0 — fixed in 140.15.0, 153.2.0, 155.0

How to fix

Upgrade to a fixed version: Mozilla Thunderbird 140.15.0, 153.2.0, 155.0. Check the vendor advisory for mitigations.

References

https://bugzilla.mozilla.org/show_bug.cgi?id=2057805
https://www.mozilla.org/security/advisories/mfsa2026-86/
https://www.mozilla.org/security/advisories/mfsa2026-87/
https://www.mozilla.org/security/advisories/mfsa2026-88/

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