CVE-2026-84143 Internally found bugs fixed in Thunderbird 155, Thunderbird ESR 153.2 and Thunderbird ESR 140.15

Critical 9.8 Published 1 Sep 2026 Updated 3 Sep 2026

Memory corruption and information disclosure flaws in Thunderbird allow attackers to potentially execute code or leak data, requiring an upgrade to patched versions.

Exploited in the wild: Not known
Login needed: Not needed
Attackable: Over the internet
Victim action needed: Not needed
Fix: Available

What is CVE-2026-84143?

CVE-2026-84143 is a critical memory corruption vulnerability in Mozilla Firefox, published on 1 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

Official description

Internally found bugs present in Thunderbird 154, Thunderbird ESR 153.1 and Thunderbird ESR 140.14. Some of these bugs showed evidence of memory corruption or another security-relevant defect and we presume that with enough effort some of these could have been exploited. This vulnerability was fixed in Firefox 155, Firefox ESR 140.15, Firefox ESR 153.2, Thunderbird 155, Thunderbird 140.15, and Thunderbird 153.2.

Weakness: CWE-119, CWE-200

Which versions are affected by CVE-2026-84143?

It affects 2 products.

Products include Mozilla Firefox, Mozilla Thunderbird.

How do I fix CVE-2026-84143?

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-2026-84143 being exploited?

There is no known exploitation of CVE-2026-84143 in the wild and no public exploit code.

All Mozilla Firefox vulnerabilities

Sources

Also known as DEBIAN-CVE-2026-84143, GHSA-hcr8-q9p9-c49f, UBUNTU-CVE-2026-84143.