The Fivestar module 6.x-1.x before 6.x-1.20 for Drupal does not properly validate voting data, which allows remote attackers to manipulate voting averages via a negative value in the vote parameter.
http://www.securityfocus.com/bid/52984
http://www.openwall.com/lists/oss-security/2012/04/12/2
http://www.openwall.com/lists/oss-security/2012/04/11/4
http://secunia.com/advisories/48788