What is CVE-2026-84640?

CVE-2026-84640 is a high-severity vulnerability in Mozilla Thunderbird, classified under Buffer Over-read. CVSS score: 7.5/10. Published 2026-09-01.

How severe is CVE-2026-84640?

High severity. CVSS v3 base score is 7.5 out of 10.

Vulnerability in Mozilla Thunderbird

CVE-2026-84640

A maliciously constructed mail header could lead to a one byte read past the end of a buffer. 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 7.5 (High). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N.

EPSS exploit prediction

EPSS: 0.003 (16.9th percentile), scored 2026-10-17.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 17th percentile — 16.9% 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-126 — Buffer Over-read

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-2026-84640?: CVE-2026-84640 is a high-severity vulnerability in Mozilla Thunderbird, classified under Buffer Over-read. CVSS score: 7.5/10. Published 2026-09-01.
How severe is CVE-2026-84640?: High severity. CVSS v3 base score is 7.5 out of 10.