source: http://www.securityfocus.com/bid/152/info

A vulnerability exists in the WWW Authorization Gateway program written by Ray Chan. Version 1.0 fails to eliminate characters with special meaning to the shell prior to executing a command. As a result, an attacker can utilize certain characters to execute arbitrary commands on a system remotely, as whatever user invoked the cgi-bin.

Place the following as a username: 
| some command 
and any password. The command will be executed.