fetchmail -- TLS enforcement problem/MITM attack/password exposure

Matthias Andree reports:

Fetchmail has had several longstanding password disclosure
vulnerabilities.

sslcertck/sslfingerprint options should have implied
“sslproto tls1” in order to enforce TLS negotiation,
but did not.
Even with “sslproto tls1” in the config, fetches
would go ahead in plain text if STLS/STARTTLS wasn’t available
(not advertised, or advertised but rejected).
POP3 fetches could completely ignore all TLS options
whether available or not because it didn’t reliably issue
CAPA before checking for STLS support - but CAPA is a
requisite for STLS. Whether or not CAPAbilities were probed,
depended on the “auth” option. (Fetchmail only
tried CAPA if the auth option was not set at all, was set
to gssapi, kerberos, kerberos_v4, otp, or cram-md5.)
POP3 could fall back to using plain text passwords, even
if strong authentication had been configured.
POP2 would not complain if strong authentication or TLS
had been requested.