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Mozilla Thunderbird CVE-2026-84639

| EUVDEUVD-2026-69708 CRITICAL
Use of Uninitialized Variable (CWE-457)
2026-09-01 mozilla GHSA-4x6j-cpvp-7f2c
Information Disclosure Mozilla Thunderbird Suse
9.1
CVSS 3.1 · Vendor: mozilla
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Severity by source

Vendor (mozilla) PRIMARY
9.1 CRITICAL
AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:H
vuln.today AI
8.1 HIGH

Remote crafted email, no auth; assessed UI:R because rendering the attacker's message is required, but C:H (memory leak) and A:H (crash) retained, I:N.

3.1 AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:N/A:H
4.0 AV:N/AC:L/AT:N/PR:N/UI:P/VC:H/VI:N/VA:H/SC:N/SI:N/SA:N
SUSE
HIGH
qualitative

Primary rating from Vendor (mozilla).

CVSS VectorVendor: mozilla

Attack Vector
Network
Attack Complexity
Low
Privileges Required
None
User Interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
None
Availability
High

Lifecycle Timeline

4
Analysis Generated
Sep 02, 2026 - 19:29 vuln.today
CVSS changed
Sep 02, 2026 - 19:22 NVD
9.1 (CRITICAL)
CVE Published
Sep 01, 2026 - 21:33 cve.org
UNKNOWN (no severity yet)
CVE Published
Sep 01, 2026 - 21:33 cve.org
CRITICAL 9.1

DescriptionCVE.org

Triggering an error condition in certain MIME bodies would cause uninitialized memory to be used. This vulnerability was fixed in Thunderbird 155, Thunderbird 140.15, and Thunderbird 153.2.

AnalysisAI

Use of uninitialized memory in Mozilla Thunderbird's MIME parser allows a remote attacker who sends a specially crafted email to trigger an error condition in certain MIME bodies, causing the client to read uninitialized memory. Depending on what previously occupied that memory, this can leak sensitive process data or crash the application. …

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Attack ChainAIDerived

Hypothetical attack flow derived from CVE metadata

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Vulnerability AssessmentAI

Exploitation Exploitation requires the victim's Thunderbird client to parse a specially crafted email whose MIME body triggers a specific error condition in the MIME handling code - the malformed/error-inducing MIME structure IS the required condition. … Additional conditions and limiting factors are described in the full assessment.
Risk Assessment The CVSS 3.1 vector CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:H (base 9.1) describes a network-reachable, unauthenticated, no-user-interaction attack with high confidentiality and availability impact but no integrity impact - appropriate for an email client that auto-renders message content in a preview pane. … Full risk analysis with EPSS, KEV, and SSVC signal comparison available after sign-in.
Exploit Scenario Full exploit scenario with step-by-step reproduction available after sign-in.
Remediation Vendor-released patch: upgrade to Thunderbird 155, or to 140.15 if you are on the 140 ESR branch, or to 153.2 on the 153 branch - whichever matches your deployment channel - as documented in Mozilla advisories mfsa2026-86/87/88. … Detailed patch versions, workarounds, and compensating controls in full report.
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Within 24 hours, identify all systems running Thunderbird and document current versions deployed across your organization. …

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Vendor StatusVendor

SUSE

Severity: Important
Product Status
openSUSE Tumbleweed Fixed
SUSE Linux Enterprise Desktop 15 SP7 Affected
SUSE Linux Enterprise Desktop 15 SP7 Affected
SUSE Linux Enterprise Module for Desktop Applications 15 SP7 Affected
SUSE Linux Enterprise Module for Package Hub 15 SP7 Affected

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CVE-2026-84639 vulnerability details – vuln.today
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