CVE-2026-84640CWE-126

CVE-2026-84640

High · published September 2, 2026

CVSS v3.1

7.5

EPSS

Percentile

17.9

In the wild

Unconfirmed

What it is

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.

The record

Technical detail

CVSS v3.1: 7.5 · HIGH
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N
CVSS v4.0: Not supplied
EPSS: 0.00263 · 17.9th percentile
Weakness: CWE-126 · Buffer Over-read
Published: 2026-09-02T02:17Z

Affected products (3)

Product	Versions	Fixed in
mozilla/thunderbird	< 140.15.0	140.15.0
mozilla/thunderbird	≥ 141.0, < 153.2.0	153.2.0
mozilla/thunderbird	≥ 154.0, < 155.0	155.0

References (4)

https://bugzilla.mozilla.org/show_bug.cgi?id=2063964 · [email protected]https://www.mozilla.org/security/advisories/mfsa2026-86/ · [email protected]https://www.mozilla.org/security/advisories/mfsa2026-87/ · [email protected]https://www.mozilla.org/security/advisories/mfsa2026-88/ · [email protected]

EPSS history

Timeline

03 SEP 03:32Z
EPSS moved — → 0%
epss
01 SEP 21:33Z
One byte overflow read in mail parser
cvelistv5