Cross-site scripting vulnerability in YaBB.cgi for Yet Another Bulletin Board (YaBB) 1 Gold SP1 and earlier allows remote attackers to execute arbitrary script as other web site visitors via script in the num parameter, which is not filtered in the resulting error message.
http://www.iss.net/security_center/static/9408.php
http://archives.neohapsis.com/archives/bugtraq/2002-06/0261.html