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.
https://oval.cisecurity.org/repository/search/definition/oval%3Aorg.mitre.oval%3Adef%3A5757
https://oval.cisecurity.org/repository/search/definition/oval%3Aorg.mitre.oval%3Adef%3A11070
https://exchange.xforce.ibmcloud.com/vulnerabilities/53000
http://www.securityfocus.com/bid/36368
http://www.openwall.com/lists/oss-security/2009/08/24/2
http://secunia.com/advisories/37071