Critical severity · published Sep 1, 2026
CVE-2026-84639
Uninitialized memory in MIME parsing
9.1
CVSS
0.3%
EPSS
No
On KEV
Description
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.
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:H
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Confidentiality
High
Integrity
None
Availability
High
Scope
Unchanged
Exploitation
Is it being used?
Exploit prediction (EPSS)0.3% · higher than 25% of CVEs
Exploited in the wildNo evidence yet
CISA KEVNot listed
Ransomware campaign useNot known
Affected products
Which versions
TH
Thunderbird · Mozilla
before 140.15.0; 141.0 before 153.2.0; 154.0 before 155.0 · source: NVD
References
NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-84639
CVE.org
https://www.cve.org/CVERecord?id=CVE-2026-84639
Permissions Required
https://bugzilla.mozilla.org/show_bug.cgi?id=2061087
Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2026-86/
Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2026-87/
Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2026-88/
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