source: http://www.securityfocus.com/bid/29662/info

Flat Calendar is prone to multiple authentication-bypass vulnerabilities because it fails to perform adequate authentication checks.

An attacker can exploit these issues to gain unauthorized access to the application and make arbitrary changes to its configuration. This may lead to further attacks.

Flat Calendar 1.1 is vulnerable; other versions may also be affected. 

http://www.example.com/calender_path/admin/add.php
http://www.example.com/calender_path/admin/deleteEvent.php?eventNumber=[EVENTNUMBERid]