If your OS allows that, you need a better OS. Seriously, that's practically like having an OS that lets anyone on your LAN just SSH into your machine as root without a password just because they happen to be on the same subnet.
$ man dma
No manual entry for dmaAnd the description of how to exploit it
http://www.schneier.com/blog/archives/2006/06/hacking_comput...
The FireWire version http://www.storm.net.nz/static/files/ab_firewire_rux2k6-fina...