The Wocky module in Telepathy Gabble before 0.16.6 and 0.17.x before 0.17.4, when connecting to a "legacy Jabber server," does not properly enforce the WockyConnector:tls-required flag, which allows remote attackers to bypass TLS verification and perform a man-in-the-middle attacks.
https://bugs.freedesktop.org/show_bug.cgi?id=65036
http://www.ubuntu.com/usn/USN-1873-1
http://www.debian.org/security/2013/dsa-2702