What’s Vulnerable › Vulnerabilities

CVE-2026-84144: Internally found bugs fixed in Thunderbird 155 and Thunderbird ESR 153.2

• Severity: high (CVSS 7.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.4% 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 and Thunderbird ESR 153.1. 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 153.2, Thunderbird 155, and Thunderbird 153.2.

Affected products

• Mozilla Firefox: before 153.2.0; from 154.0.0 before 155.0.0 — fixed in 153.2.0, 155.0.0
• Mozilla Thunderbird: before 153.2.0; from 154.0 before 155.0 — fixed in 153.2.0, 155.0

How to fix

Upgrade to a fixed version: Mozilla Firefox 153.2.0, 155.0.0; Mozilla Thunderbird 153.2.0, 155.0. Check the vendor advisory for mitigations.

References

• https://bugzilla.mozilla.org/buglist.cgi?bug_id=2054619%2C2054620%2C2054624%2C2054702%2C2054726%2C2054775%2C2055703%2C2058006%2C2058013%2C2058098%2C2058627%2C2058661%2C2059127%2C2059128%2C2059180%2C2059199%2C2059205%2C2061320%2C2061430%2C2061495%2C2061505%2C2061532%2C2061799
• https://bugzilla.mozilla.org/buglist.cgi?bug_id=2054625%2C2059002%2C2061775
• https://bugzilla.mozilla.org/buglist.cgi?bug_id=2054691%2C2058085%2C2059144%2C2059191%2C2059192%2C2061521%2C2062395%2C2062404
• https://www.mozilla.org/security/advisories/mfsa2026-82/
• https://www.mozilla.org/security/advisories/mfsa2026-85/
• https://www.mozilla.org/security/advisories/mfsa2026-86/
• https://www.mozilla.org/security/advisories/mfsa2026-88/

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