IRC Xchat client versions 1.4.2 and earlier allows remote attackers to execute arbitrary commands by encoding shell metacharacters into a URL which XChat uses to launch a web browser.
http://www.securityfocus.com/bid/1601
http://www.redhat.com/support/errata/RHSA-2000-055.html
http://archives.neohapsis.com/archives/bugtraq/2000-08/0305.html
http://archives.neohapsis.com/archives/bugtraq/2000-08/0301.html
http://archives.neohapsis.com/archives/bugtraq/2000-08/0215.html