source: http://www.securityfocus.com/bid/15662/info

WebCalendar is prone to multiple SQL injection vulnerabilities.

This vulnerability could permit remote attackers to pass malicious input to database queries, resulting in modification of query logic or other attacks.

Successful exploitation could result in a compromise of the application, disclosure or modification of data, or may permit an attacker to exploit vulnerabilities in the underlying database implementation.

Version 1.0.1. is vulnerable; other versions may also be affected. 

http://www.example.com/webcalendar/activity_log.php?startid=%2527
http://www.example.com/webcalendar/activity_log.php?startid=%27
http://www.example.com/webcalendar/activity_log.php?startid='