source: http://www.securityfocus.com/bid/15673/info

WebCalendar is prone to an HTTP response-splitting vulnerability. This issue is due to a failure in the application to properly sanitize user-supplied input.

A remote attacker may exploit this vulnerability to influence or misrepresent how web content is served, cached, or interpreted. This could aid in various attacks that attempt to entice client users into a false sense of trust.

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

http://www.example.com/webcalendar/layers_toggle.php?status=on&ret=[url_redirect_to]