9.1Critical
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.
What this means for your business
- It affects Mozilla Thunderbird. It matters if your company, or a supplier that handles your data, runs it.
- An attacker can use it remotely, over a network, without a login, and without anyone at your company clicking anything.
- FIRST's prediction model gives it a 0.6% chance of attack attempts being seen in the next 30 days, ranking above 43% of all known flaws.
What to do
- 1Check whether your company or your suppliers run Mozilla Thunderbird, and which version. The affected versions are listed further down this page.
- 2If you do, apply the vendor's fix. A patch or vendor advisory has been published.
Fastnexa security experts
Not sure if your company is exposed to CVE-2026-84639?
Tell us where you run Mozilla Thunderbird and a Fastnexa penetration tester will check whether this flaw, or others like it, can be used against your websites, apps and network.
The full test is free for our first 10 founding clients until 31 December 2026. See the offer
Think you’ve already been hit? Don’t wait on a form: call or WhatsApp +1 (732) 454 2616. We reply within 1 hour, 24/7. Emergency help →
Scoring
- CVSS
- 9.1 (v3.1)
- Vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:H- Weakness
- CWE-457
- Assigned by
- security@mozilla.org
Dates
- Published
- 2026-09-01
- Last modified
- 2026-09-03
- Sources
- NVD
Affected products
- Mozilla Thunderbird- 140.15.0, 141.0 - 153.2.0, 154.0 - 155.0
As listed in the NVD configuration data. Not a statement about your estate.