Since deep space probes and satellites receive commands from Earth via radio, what prevents unauthorized commands from being accepted by such devices?
For deep space probes, the required power of radio transmissions would seem to provide some degree of security since sufficiently capable transmitters would have some security to prevent unauthorized use. (The cost of building such a transmitter would also make building one's own relatively unattractive for limited use even if the project site was in a country with weak enforcement of regulations about use of radio frequencies. The cost and risk of building a transmitter relative to the payoff would probably discourage most crackers [doing such for fun or to prove their cleverness] and terrorists.)
Lack of public documentation of the systems would tend to increase the difficulty of sending meaningful commands to such systems. (A denial of service attack might be somewhat easier than a hijacking attack given such limited information while still being noteworthy.)
Because of processing power and energy use limits, it seems unlikely that early deep space probes used any form of encryption even for command and control transmissions (beyond a perhaps undocumented form of error detection and correction encoding), but this would seem to be less of an issue for more recently deployed systems.
While such systems are probably not considered high value targets by terrorists or system crackers (excluding military [and perhaps communications] satellites, which presumably do use encryption), even a temporary disruption of a scientific probe might be sufficiently newsworthy to attract some attempts.
This question was inspired by Do any spacecraft use carkeys?.