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?
There are several problems if you want to interfere that communication.
First, you need to reverse engineer the protocol. For that you need to be able to log what nasa is sending and what they are receiving. Both has quite some problems, as the sending signal is tightly focused towards the spacecraft, so you probably have to find the actual dish and install a bug to read what they are sending. The NASA might object to that.
Second, you would need the hardware. Which is beyond the normal hobbyists budget.
And finally, you would have to be 100% spot on in your reverse engineering of the protocol or you’ll probably just brick that thing.
A state actor could definitely do those things. The question is why though. There’s not really anything to gain from that, they’d just lose money on it.
Some people just like to watch the world burn
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.
I imagine it’s classified, but NASA being NASA, you know a full risk assessment along these lines exists, probably with dutiful updates as it’s still an active program. It would be fascinating read especially with the updates, with how much the world has changed while the probe remained the same.