CVE-2021-29969: Mozilla Thunderbird

Medium severity, CVSS 5.9. EPSS: 1.2% chance of exploitation in the next 30 days.

If Thunderbird was configured to use STARTTLS for an IMAP connection, and an attacker injected IMAP server responses prior to the completion of the STARTTLS handshake, then Thunderbird didn't ignore the injected data. This could have resulted in Thunderbird showing incorrect information, for example the attacker could have tricked Thunderbird to show folders that didn't exist on the IMAP server. This vulnerability affects Thunderbird < 78.12.

Affected products

• Mozilla Thunderbird: before 78.12 (fixed in 78.12)

Published 2021-08-05. Last modified 2026-06-17.