last 2008 Cyber attack on US defense network was the worst one, it was propagated thru infected USB Thumbdrive
Under Worm Assault, Military Bans Disks, USB Drives | Danger Room | Wired.com
Pentagon: All options on table in cyber-attack - Yahoo! News
just like i said.... most cyber assault had insider to help the propagate the worm to command and control the targeted network..