fetchmail -- STARTTLS denial of service

Matthias Andree reports:

Fetchmail version 5.9.9 introduced STLS support for POP3,
version 6.0.0 added STARTTLS for IMAP. However, the actual
S(TART)TLS-initiated in-band SSL/TLS negotiation was not guarded
by a timeout.
Depending on the operating system defaults as to TCP stream
keepalive mode, fetchmail hangs in excess of one week after
sending STARTTLS were observed if the connection failed without
notifying the operating system, for instance, through network
outages or hard server crashes.
A malicious server that does not respond, at the network level,
after acknowledging fetchmail’s STARTTLS or STLS request, can
hold fetchmail in this protocol state, and thus render fetchmail
unable to complete the poll, or proceed to the next server,
effecting a denial of service.
SSL-wrapped mode on dedicated ports was unaffected by this
problem, so can be used as a workaround.