ASMAX 804 gu router is a SOHO class device. It provides ADSL / WiFi / Ethernet interfaces.
There is an unauthenticated maintenance script (named 'script') in /cgi-bin/ directory of the web management interface.
When 'system' paramether is passed to the script it allows running OS shell commands (as root).
PoC: GET request to: http://192.168.1.1/cgi-bin/script?system%20whoami
Using CSRF attack one could remotely own a router using for example simple <img> html tags pointing to http://192.168.1.1/...
The issue was tested on firmware: 66.34.1
The vendor was notified on 30.12.08, but we got no reasonable response till now (the bug remains unpatched).
More information: http://www.securitum.pl/dh/asmax-ar-804-gu-compromise