protocols/jabber/auth.c in libpurple in Pidgin 2.6.0, and possibly other versions, does not follow the “require TLS/SSL” preference when connecting to older Jabber servers that do not follow the XMPP specification, which causes libpurple to connect to the server without the expected encryption and allows remote attackers to sniff sessions.
bugs.debian.org/cgi-bin/bugreport.cgi?bug=542891
developer.pidgin.im/ticket/8131
developer.pidgin.im/viewmtn/revision/diff/312e056d702d29379ea61aea9d27765f127bc888/with/55897c4ce0787edc1e7721b7f4a9b5cbc8357279
secunia.com/advisories/37071
www.openwall.com/lists/oss-security/2009/08/24/2
www.securityfocus.com/bid/36368
exchange.xforce.ibmcloud.com/vulnerabilities/53000
oval.cisecurity.org/repository/search/definition/oval%3Aorg.mitre.oval%3Adef%3A11070
oval.cisecurity.org/repository/search/definition/oval%3Aorg.mitre.oval%3Adef%3A5757