Its partially because there is only one set of antennas large enough to communicate with it, and that's only sometimes. Its called the Deep space network and it is very secure because it's used for many things, not just communicating with the Voyager probes.

Second, you'd have to have very very intimate knowledge of the hardware, and programming language to even begin to hack it. And the people who do have that knowledge are very very passionate about their probes.

So I guess technically the answer is yes. But practically, no.

I think the security is adequately managed by the need for a massive transmitter as well as the question "what is there to gain via a hostile takeover and re-programming the probe?"

I bet there's actual security of some kind going on, but those two points seem like a massive hurdle to clear just to mess with a deep space probe.

Modern satellites are protected by various means of encryption, but there’s an enthusiast community that tracks down and communicates with very old unencrypted zombie satellites. There’s even been an NGO which managed to fire rockets on an abandoned NASA/ESA probe (with their approval.)

The Voyagers benefits primarily from the lack of groups with an adequate deep space network to communicate with it. Their communication standards are otherwise completely open and well documented.