source: https://www.securityfocus.com/bid/14504/info

Calendar Express is prone to multiple SQL-injection vulnerabilities because the application fails to properly sanitize user-supplied input before using it in SQL queries.

A successful exploit could allow an attacker to compromise the application, access or modify data, or exploit vulnerabilities in the underlying database.

http://www.example.com/calendarexpress/subscribe.php?cid='