The calender.pl and the calendar_admin.pl calendar scripts by Matt Kruse allow remote attackers to execute arbitrary commands via shell metacharacters.
https://euvd.enisa.europa.eu/vulnerability/EUVD-2000-0431
http://www.securityfocus.com/bid/1215
http://archives.neohapsis.com/archives/bugtraq/2000-05/0173.html