The XMLRPC API in WordPress before 3.5.1 allows remote attackers to send HTTP requests to intranet servers, and conduct port-scanning attacks, by specifying a crafted source URL for a pingback, related to a Server-Side Request Forgery (SSRF) issue.
https://bugzilla.redhat.com/show_bug.cgi?id=904120
http://www.acunetix.com/blog/web-security-zone/wordpress-pingback-vulnerability/