Piwik before 1.1 does not properly limit the number of files stored under tmp/sessions/, which might allow remote attackers to cause a denial of service (inode consumption) by establishing many sessions.
https://exchange.xforce.ibmcloud.com/vulnerabilities/64638
http://www.securityfocus.com/bid/45787
http://piwik.org/blog/2011/01/piwik-1-1-2/