The DisplayTopKeywords function in plugins/Referrers/Controller.php in Piwik before 2.15.0 allows remote attackers to conduct PHP object injection attacks, conduct Server-Side Request Forgery (SSRF) attacks, and execute arbitrary PHP code via a crafted HTTP header.
https://piwik.org/changelog/piwik-2-15-0/
http://www.securityfocus.com/archive/1/536839/100/0/threaded