The fact that Microsoft Windows will automatically run software from a USB drive with no user intervention is a well-known security vulnerability. For example, the autorun function is the way that the infamous Sony rootkit gets its hooks into your system. With this feature enabled (or, rather, not blocked) on PCs, its an easy way for malware to be installed on your desktops via USB. Read this chilling example:
Dark Reading - Host security - Social Engineering, the USB Way - Security:
... Once I seeded the USB drives, I decided to grab some coffee and watch the employees show up for work. Surveillance of the facility was worth the time involved. It was really amusing to watch the reaction of the employees who found a USB drive. You know they plugged them into their computers the minute they got to their desks.
I immediately called my guy that wrote the Trojan and asked if anything was received at his end. Slowly but surely info was being mailed back to him. I would have loved to be on the inside of the building watching as people started plugging the USB drives in, scouring through the planted image files, then unknowingly running our piece of software.
After about three days, we figured we had collected enough data. When I started to review our findings, I was amazed at the results. Of the 20 USB drives we planted, 15 were found by employees, and all had been plugged into company computers. The data we obtained helped us to compromise additional systems, and the best part of the whole scheme was its convenience. We never broke a sweat. Everything that needed to happen did, and in a way it was completely transparent to the users, the network, and credit union management.
Of all the social engineering efforts we have performed over the years, I always had to worry about being caught, getting detained by the police, or not getting anything of value. The USB route is really the way to go. With the exception of possibly getting caught when seeding the facility, my chances of having a problem are reduced significantly.
You’ve probably seen the experiments where users can be conned into giving up their passwords for a chocolate bar or a $1 bill. But this little giveaway took those a step further, working off humans' innate curiosity. Emailed virus writers exploit this same vulnerability, as do phishers and their clever faux Websites. Our credit union client wasn’t unique or special. All the technology and filtering and scanning in the world won’t address human nature. But it remains the single biggest open door to any company’s secrets.
Disagree? Sprinkle your receptionist's candy dish with USB drives and see for yourself how long it takes for human nature to manifest itself.
Also read Bruce Schneier on this avenue of attack: Schneier on Security: Hacking Computers Over USB.