CVE-2026-4371: Mozilla Thunderbird

High severity, CVSS 7.4. EPSS: 0.6% chance of exploitation in the next 30 days.

A malicious mail server could send malformed strings with negative lengths, causing the parser to read memory outside the buffer. If a mail server or connection to a mail server were compromised, an attacker could cause the parser to malfunction, potentially crashing Thunderbird or leaking sensitive data. This vulnerability was fixed in Thunderbird 149 and Thunderbird 140.9.

Affected products

• Mozilla Thunderbird: before 140.9.0 (fixed in 140.9.0); before 149.0 (fixed in 149.0)

Published 2026-03-24. Last modified 2026-07-15.