The Security Mentor

Reports are mixed right now, but it is definitely possible to shut down Internet Connection Sharing by sending it bad data over the network. Some reports are that it takes the firewall down with it. Other reports are that the bad data can bring it down even from outside the firewall.

Buy and install one of those $30 firewall boxes from Circuit City or your office supply store and you won't have to worry about this. If you're already running behind a firewall appliance like that, you can safely ignore the entire story.

Semi-technical details at Network World and a volunteer security organization