Fetchmail before 6.4.22 fails to enforce STARTTLS session encryption in some circumstances, such as a certain situation with IMAP and PREAUTH.
Created fetchmail tracking bugs for this issue:
Affects: fedora-all [bug 1999191]
This specific flaw is caused by a myriad of factors that result in the possibility of communications being sent in clear text. Due to how IMAP and fetchmail work, it was possible for fetchmail to start communications in clear text and then try to upgrade the communication to an encrypted communication (STARTTLS protocol). However, IMAP in "Pre Auth" mode does not allow for the communications to be started via STARTTLS. Thus if a user used IMAP, the server used Pre Auth, TLS was forced and no ssl mode was configured, the communications would be sent entirely in clear text. This has been solved in Fetchmail 6.4.22 or above.