What’s Vulnerable › Vulnerabilities

CVE-2021-4126

• Severity: medium (CVSS 6.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.5% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2022-12-22 · updated 2026-06-17

Description

When receiving an OpenPGP/MIME signed email message that contains an additional outer MIME message layer, for example a message footer added by a mailing list gateway, Thunderbird only considered the inner signed message for the signature validity. This gave the false impression that the additional contents were also covered by the digital signature. Starting with Thunderbird version 91.4.1, only the signature that belongs to the top level MIME part will be considered for the displayed status. This vulnerability affects Thunderbird < 91.4.1.

Affected products

• Mozilla Thunderbird: before 91.4.1 — fixed in 91.4.1

How to fix

Upgrade to a fixed version: Mozilla Thunderbird 91.4.1. Check the vendor advisory for mitigations.

References

• https://bugzilla.mozilla.org/show_bug.cgi?id=1732310
• https://www.mozilla.org/security/advisories/mfsa2021-55/
• https://bugzilla.mozilla.org/show_bug.cgi?id=1732310
• https://www.mozilla.org/security/advisories/mfsa2021-55/

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