The ssl_do_connect function in common/server.c in HexChat before 2.10.2, XChat, and XChat-GNOME does not verify that the server hostname matches a domain name in the X.509 certificate, which allows man-in-the-middle attackers to spoof SSL servers via an arbitrary valid certificate.
https://github.com/hexchat/hexchat/issues/524
https://github.com/hexchat/hexchat/commit/c9b63f7f9be01692b03fa15275135a4910a7e02d
https://bugzilla.redhat.com/show_bug.cgi?id=1081839