source: https://www.securityfocus.com/bid/15917/info

WebCal is prone to multiple HTML injection and cross-site scripting vulnerabilities. These issues are due to a failure in the application to properly sanitize user-supplied input before using it in dynamically generated content.

Attacker-supplied HTML and script code would be executed in the context of the affected Web site, potentially allowing for theft of cookie-based authentication credentials. An attacker could also exploit the HTML injection issue to control how the site is rendered to the user; other attacks are also possible.

These issues affect version 3.04; earlier versions may also be vulnerable. 

http://www.example.com/perl/webcal.cgi?function=<script>alert(document.cookie)</script>&cal=public
http://www.example.com/perl/webcal.cgi?function=webyear&cal=public&year=<script>alert(document.cookie)</script>
http://www.example.com/perl/webcal.cgi?function=webday&cal=public&date=<script>alert(document.cookie)</script>