Department of Defense official says classified network was compromised in 2008 through an infected USB drive. Security community has been wary of USB drives for some time now, with some IT departments going so far as to permanently disable the USB ports on users machines. Pentagon famously ordered a complete ban of the devices a couple of years ago because of an unspecified security incident. USB drives represent a serious threat, especially for uneducated users who don’t understand the ways in which they can be used to spread malicious code and siphon off valuable data.
Source: https://threatpost.com/usb-drives-remain-major-security-threat-090110/74412/