CVE-2026-84142: Internally found bugs fixed in Thunderbird 155

Severity: critical (CVSS 9.8 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.6% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-09-01 · updated 2026-09-03
Weakness: CWE-119

Description

Internally found bugs present in Thunderbird 154. 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 and Thunderbird 155.

Affected products

Mozilla Firefox: before 155.0.0 — fixed in 155.0.0
Mozilla Thunderbird: before 155.0 — fixed in 155.0

How to fix

Upgrade to a fixed version: Mozilla Firefox 155.0.0; Mozilla Thunderbird 155.0. Check the vendor advisory for mitigations.

References

https://bugzilla.mozilla.org/buglist.cgi?bug_id=2029421%2C2040889%2C2045313%2C2045435%2C2053578%2C2058621%2C2058626
https://bugzilla.mozilla.org/buglist.cgi?bug_id=2048796%2C2053150%2C2057356
https://www.mozilla.org/security/advisories/mfsa2026-82/
https://www.mozilla.org/security/advisories/mfsa2026-86/

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