libpurple in Pidgin before 2.2.1 does not properly handle MSN nudge messages from users who are not on the receiver's buddy list, which allows remote attackers to cause a denial of service (crash) via a nudge message that triggers an access of "an invalid memory location."
https://oval.cisecurity.org/repository/search/definition/oval%3Aorg.mitre.oval%3Adef%3A18261
https://exchange.xforce.ibmcloud.com/vulnerabilities/36884
http://www.vupen.com/english/advisories/2007/3321
http://www.securityfocus.com/bid/25872
http://www.securityfocus.com/archive/1/481402/100/0/threaded
http://www.pidgin.im/news/security/?id=23
http://secunia.com/advisories/27088