The mars rover, like most other things man has directed into outer space, receives, faithfully executes, and responds to instructions sent to it from its controllers on Earth.
Presumably (and hopefully) it takes more than thorough knowledge of specific communication protocols and sufficient broadcast power to take control of the mars rover, Hubble telescope, or any specific orbiting satellite for that matter.
What manner of security is is place to protect the information going to and from satellites, vessels, and rovers, and how has this security come along throughout history?