I mean the Voyager 1 probe which is currently the human-made object the farthest away from earth. The space program people operating the mission seem to have great control options, they even “moved software from one chip to another” (link) Apart from the probably gigantic and expensive installation needed to receive and/or send messages from/to that far away from home (23 hours of delay?), are there any safety measures to prevent a potentially malicous actor from sending commands to the probe?
This is probably the biggest problem with that idea. If you hack an ancient space probe, you’ll definitely get the appropriate bragging rights, but that’s about it.
Lots of hobby hackers would love to do it, but they don’t have the hardware for it. Some governments might have the hardware, but they just don’t care about bragging that much.