Flat Calendar 1.1 does not properly restrict access to administrative functions, which allows remote attackers to (1) add new events via calAdd.php, as reachable from admin/add.php, or (2) delete events via admin/deleteEvent.php. NOTE: this is only a vulnerability when the administrator does not follow recommendations in the product's security documentation.
https://exchange.xforce.ibmcloud.com/vulnerabilities/43039
http://www.securityfocus.com/archive/1/493278/100/0/threaded