It was discovered that WebCalendar, a PHP-based calendar application, performs insufficient sanitising in the exports handler, which allows injection of web script.
For the old stable distribution (sarge) this problem has been fixed in version 0.9.45-4sarge7.
The stable distribution (etch) no longer contains WebCalendar packages.
For the unstable distribution (sid) this problem has been fixed in version 1.0.5-2.
We recommend that you upgrade your webcalendar package.