Thunderbird before version 91.2 ignored the configuration to require STARTTLS security for an SMTP connection. A man-in-the-middle (MITM) could perform a downgrade attack to intercept transmitted messages, or could take control of the authenticated session to execute SMTP commands chosen by the MITM. If an unprotected authentication method was configured, the MITM could obtain the authentication credentials, too.
Thunderbird before version 91.2 ignored the configuration to require STARTTLS security for an SMTP connection. A man-in-the-middle (MITM) could perform a downgrade attack to intercept transmitted messages, or could take control of the authenticated session to execute SMTP commands chosen by the MITM. If an unprotected authentication method was configured, the MITM could obtain the authentication credentials, too.
https://www.mozilla.org/security/advisories/mfsa2021-47/ https://bugzilla.mozilla.org/show_bug.cgi?id=1733366
Vulmon