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.
Published: 2026-09-01
Score: 7.5 High
EPSS: < 1% Very Low
KEV: No
Impact: Information Disclosure
Action: Patch
AI Analysis

Impact

A malicious IMAP server can send a specially crafted ID response that causes Thunderbird to experience a use‑after‑free condition. This frees heap memory prematurely and allows the client to read the freed memory region. The leaked data can then be written to the user’s prefs.js configuration file, revealing sensitive information that was stored there. The root weakness is a use‑after‑free bug that leads to data disclosure.

Affected Systems

All versions of Thunderbird prior to 155, 140.15, and 153.2 are vulnerable. Updating the client to any of the patched releases removes this issue.

Risk and Exploitability

The flaw is exploitable by an attacker who can control the IMAP server to which a Thunderbird client connects. The attack path requires only a crafted server response delivered over the network. EPSS indicates a probability of exploitation below 1%, and the vulnerability is not listed in the CISA KEV catalog, but the CVSS score of 7.5 indicates high severity. The ability to trigger the bug remotely and potentially alter local configuration files makes the risk significant for any users that interface with untrusted IMAP servers.

Generated by OpenCVE AI on September 3, 2026 at 11:41 UTC.

Remediation

No vendor fix or workaround currently provided.

OpenCVE Recommended Actions

  • Update Thunderbird to a patched release (155, 140.15, or 153.2)
  • Disable IMAP authentication or restrict Thunderbird to servers that are fully trusted
  • Regularly review prefs.js for unexpected changes or added configuration parameters

Generated by OpenCVE AI on September 3, 2026 at 11:41 UTC.

Tracking

Sign in to view the affected projects.

Advisories
Source ID Title
Debian DLA Debian DLA DLA-4775-1 thunderbird security update
Debian DSA Debian DSA DSA-6483-1 thunderbird security update
History

Thu, 03 Sep 2026 19:15:00 +0000

Type Values Removed Values Added
CPEs cpe:2.3:a:mozilla:thunderbird:*:*:*:*:*:*:*:*

Thu, 03 Sep 2026 08:30:00 +0000

Type Values Removed Values Added
Metrics ssvc

{'options': {'Automatable': 'yes', 'Exploitation': 'none', 'Technical Impact': 'partial'}, 'version': '2.0.3'}

Wed, 02 Sep 2026 19:30:00 +0000

Type Values Removed Values Added
Weaknesses CWE-416
Metrics cvssV3_1

{'score': 8.3, 'vector': 'CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:L/A:H'}

 cvssV3_1

{'score': 7.5, 'vector': 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N'}

Wed, 02 Sep 2026 05:00:00 +0000

Type Values Removed Values Added
Weaknesses CWE-416

Wed, 02 Sep 2026 00:30:00 +0000

Type Values Removed Values Added
Weaknesses CWE-416

Wed, 02 Sep 2026 00:15:00 +0000

Type Values Removed Values Added
Weaknesses CWE-825
References
Metrics threat_severity

None

 cvssV3_1

{'score': 8.3, 'vector': 'CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:L/A:H'}

threat_severity

Important

Wed, 02 Sep 2026 00:00:00 +0000

Type Values Removed Values Added
First Time appeared Mozilla
Mozilla thunderbird
Vendors & Products Mozilla
Mozilla thunderbird

Tue, 01 Sep 2026 22:00:00 +0000

Type Values Removed Values Added
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.
Title Information disclosure due to malicious IMAP server response
References

Subscriptions

Mozilla Thunderbird
cve-icon MITRE

Status: PUBLISHED

Assigner: mozilla

Published:

Updated: 2026-09-02T18:47:17.033Z

Reserved: 2026-09-01T21:33:07.129Z

Link: CVE-2026-84641

cve-icon Vulnrichment

Updated: 2026-09-02T18:45:37.542Z

cve-icon NVD

Status : Analyzed

Published: 2026-09-01T22:17:19.900

Modified: 2026-09-03T19:01:11.823

Link: CVE-2026-84641

cve-icon Redhat

Severity : Important

Publid Date: 2026-09-01T21:33:07Z

Links: CVE-2026-84641 - Bugzilla

cve-icon OpenCVE Enrichment

Updated: 2026-09-03T11:45:03Z

Weaknesses