MondoBongo
Obligate Feeder Obsessed
this is known as a air gapped network. they're common place on very secure locations, but just like you pointed, humans are always the weak point.
the usb mechanism you mentioned is precisely the attack vector used by stuxnet (possibly the coolest malicious code ever written) to penetrate and sabotage Iranian nuclear accelerators.
the command and control infrastructure alone was impressive. wired has a great write up on it:
http://www.wired.com/2014/11/countdown-to-zero-day-stuxnet/
i can't find the link right now, but i read an article a while back reporting on a study of these rogue usb sticks. apparently if you drop a usb stick somewhere, there is a really good chance someone is going to pick it up and put it in their computer.
if that usb stick happens to have a company logo on it, the success rate is frighteningly high.
here is another iteration of the aforementioned attack vector:
http://www.darkreading.com/attacks-breaches/social-engineering-the-usb-way/d/d-id/1128081?
and here is the original article i was referring to:
http://www.bloomberg.com/news/artic...hacking-as-test-shows-nothing-prevents-idiocy
Staff secretly dropped computer discs and USB thumb drives in the parking lots of government buildings and private contractors. Of those who picked them up, 60 percent plugged the devices into office computers, curious to see what they contained. If the drive or CD case had an official logo, 90 percent were installed.