One problem with drones is that they are capable of being controlled by someone other than the legitimate drone operator. These are sometimes called zombie drones. The technical term for such takeovers is called repossession.
Repossession is a real threat. College students in Texas were able to repossess a government surveillance drone using equipment that cost little more than a thousand dollars. The technique used is called GPS spoofing. Spoofing involves mimicking the signals of the drone's global positioning device and by using a signal that is stronger than the commands of the drone operator taking over control of the drone. A terrorist could theoretically repossess a drone.
There are also GPS jammers that can interfere with the signals sent to operate the drone. Some think that this was the method used by Iran to bring down a U.S, drone over Iran in December. Although it has also been suggested that Iran used the same spoofing techniques as the students mentioned above. Should terrorists gain control of drones sent to attack them they could end up attacking U.S. bases. For more see this article.