What is CVE-2026-84639?

CVE-2026-84639 is a critical-severity vulnerability in Mozilla Thunderbird, classified under CWE-457. CVSS score: 9.1/10. Published 2026-09-01.

How severe is CVE-2026-84639?

Critical severity. CVSS v3 base score is 9.1 out of 10.

Vulnerability in Mozilla Thunderbird

CVE-2026-84639

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.

Published 2026-09-01 · last modified 2026-09-03.

CVSS v3 metric

CVSS v3 base score 9.1 (Critical). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:H.

EPSS exploit prediction

EPSS: 0.003 (23.7th percentile), scored 2026-10-17.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 24th percentile — 23.7% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.003 · newest: 0.003 · change: 0.000

Affected products

Mozilla Thunderbird

Weakness classification (CWE)

CWE-457

References

bugzilla.mozilla.org/show_bug.cgi (Permissions Required)
www.mozilla.org/security/advisories/mfsa2026-86/ (Vendor Advisory)
www.mozilla.org/security/advisories/mfsa2026-87/ (Vendor Advisory)
www.mozilla.org/security/advisories/mfsa2026-88/ (Vendor Advisory)

Frequently asked questions

What is CVE-2026-84639?: CVE-2026-84639 is a critical-severity vulnerability in Mozilla Thunderbird, classified under CWE-457. CVSS score: 9.1/10. Published 2026-09-01.
How severe is CVE-2026-84639?: Critical severity. CVSS v3 base score is 9.1 out of 10.