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?
The physical security is plenty at this point in time. Only another state actor could possibly have the resources to even handle the communication process in the first place, and it would be expensive with zero incentive to act.
Past that, the custom software would have to be both obtained from NASA via espionage and reverse engineered.
If those to impossible hurdles were somehow overcome, the international blowback would be enormous.