CVE-2026-84639 Uninitialized memory in MIME parsing
An attacker can exploit uninitialized memory in MIME parsing to potentially execute code or cause a denial of service in Thunderbird.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-84639?
CVE-2026-84639 is a critical 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
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.
Weakness: CWE-457
Which versions are affected by CVE-2026-84639?
It affects 1 product.
Products include Mozilla Thunderbird.
How do I fix CVE-2026-84639?
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-2026-84639 being exploited?
There is no known exploitation of CVE-2026-84639 in the wild and no public exploit code.
Other vulnerabilities in Mozilla Thunderbird
Sources
Also known as DEBIAN-CVE-2026-84639, GHSA-4x6j-cpvp-7f2c, UBUNTU-CVE-2026-84639.