CVE-2026-84639: Vulnerability in Mozilla Thunderbird
Description
CVE-2026-84639 is a critical vulnerability in Mozilla Thunderbird where triggering an error condition in certain MIME bodies causes uninitialized memory to be used. This flaw could lead to a high impact on confidentiality and availability. The issue has been fixed in Thunderbird versions 155, 140.15, and 153.2.
CVSS v3.1
Score 9.1critical
Affected software
Mozilla
Thunderbird
Weaknesses
AI-Powered Analysis
Machine-generated threat intelligence
Technical Analysis
This vulnerability involves the use of uninitialized memory when processing specific MIME bodies in Mozilla Thunderbird. Exploiting this flaw could allow an attacker to cause a denial of service or potentially leak sensitive information due to the use of uninitialized memory. The vulnerability is tracked as CWE-457 (Use of Uninitialized Variable). Mozilla has released fixes in Thunderbird versions 155, 140.15, and 153.2 to address this issue.
Potential Impact
The vulnerability has a CVSS score of 9.1 (critical), indicating a high impact on confidentiality and availability without requiring user interaction or privileges. An attacker can remotely trigger the flaw by sending specially crafted MIME content, potentially causing uninitialized memory to be used, which may lead to information disclosure or denial of service.
Mitigation Recommendations
Official fixes are available and have been released by Mozilla in Thunderbird versions 155, 140.15, and 153.2. Users and administrators should update to one of these fixed versions or later to remediate this vulnerability. No additional mitigation actions are indicated by the vendor advisory.
Technical Details
- Data Version
- 5.2
- Assigner Short Name
- mozilla
- Date Reserved
- 2026-09-01T21:33:05.937Z
- State
- PUBLISHED
- Vendor Advisory Urls
- [{"url":"https://www.mozilla.org/security/advisories/mfsa2026-86/","vendor":"Mozilla"},{"url":"https://www.mozilla.org/security/advisories/mfsa2026-87/","vendor":"Mozilla"},{"url":"https://www.mozilla.org/security/advisories/mfsa2026-88/","vendor":"Mozilla"}]
Threat ID: 6a97492dacd9273b493ed288
Added to database: 09/01/2026, 21:52:45 UTC
Last enriched: 10/17/2026, 23:28:30 UTC
Last updated: 10/17/2026, 23:28:30 UTC
Views: 74
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