The US fleet of aerial drones has been hit with a virus in the past that was very hard to remove. The virus was actually in the computer systems where the drones are controlled in the Predator and Reaper drone issue. A lost NATO stealth drone has been in the news a lot lately after it was captured in Iran.
That drone wasn't shot down; Iranian scientists claim they were able to trick the drone into landing where they wanted it using a GPS hack. The Iranians used software that spoofed GPS coordinates, and that spoof tricked the RQ-170 Sentinel into landing on its own without the Iranians needing to hack the controls. Before spoofing the GPS coordinates, the GPS signal was jammed on the drone, forcing it into autopilot.
Gizmodo reports that the military has known of the potential to spoof GPS and cause a drone to land in this fashion since 2003 and has never addressed the issue. There has been no confirmation outside of a supposed Iranian scientist that this GPS hack was what brought the drone down into Iranian hands.