CVE-2026-84640 One byte overflow read in mail parser
A malicious mail header causes a one-byte buffer over-read in Thunderbird, potentially leading to crashes or information disclosure.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-84640?
CVE-2026-84640 is a high-severity memory corruption vulnerability in Mozilla Thunderbird, published on 1 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
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.
Weakness: CWE-126
Which versions are affected by CVE-2026-84640?
It affects 1 product.
Products include Mozilla Thunderbird.
How do I fix CVE-2026-84640?
Upgrade to a fixed version.
Upgrade to the fixed version. Memory-safe languages, compiler hardening and fuzzing prevent most new cases. More about Memory corruption.
Is CVE-2026-84640 being exploited?
There is no known exploitation of CVE-2026-84640 in the wild and no public exploit code.
Other vulnerabilities in Mozilla Thunderbird
Sources
Also known as DEBIAN-CVE-2026-84640, GHSA-2q8r-2ghc-82v8, UBUNTU-CVE-2026-84640.