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/
https://euvd.enisa.europa.eu/vulnerability/EUVD-2015-7714
http://www.securityfocus.com/archive/1/536839/100/0/threaded
http://seclists.org/fulldisclosure/2015/Nov/15
http://packetstormsecurity.com/files/134220/Piwik-2.14.3-PHP-Object-Injection.html