The Christian Science Monitor reports that the Iranians hijacked the drone by using software that spoofed GPS coordinates, forcing it to land wherever they chose. An unnamed Iranian engineer is reported to have said that they reconfigured the GPS system of the RQ-170 Sentinel, forcing it to "land on its own where we wanted it to, without having to crack the remote-control signals and communications."
"The GPS navigation is the weakest point," he continued. "By putting noise [jamming] on the communications, you force the bird into autopilot. This is where the bird loses its brain."
That simple hack was effective enough for Iranians to get their hands on the drone. According to a report cited by The Register, military officials have known about the craft's GPS vulnerability since 2003. The report states:
A more pernicious attack involves feeding the GPS receiver fake GPS signals so that it believes it is located somewhere in space and time that it is not... This 'spoofing' attack is more elegant than jamming because it is surreptitious.