I got a kick out of this one, basically the hackers cut a hole into the ATM near where the PC (yes its usually a desktop running windows) is located. And then insert a thumb drive and compromise the box. How?
My theory is they run some sort of USB hacksaw like program that compromises the box.
http://www.ubergizmo.com/2013/12/atm-hack-uses-usb-drives-to-steal-money/?utm_source=mainrss