Nobody is going to hack a pinball machine. There's no point to it. People can't even hack into games that are in their possession, let alone through the internet. There has to be some sort of money aspect before it would be a possibility, and there isn't. Our hobby just isn't that important. Everyone else is off doing hacks for the 1000's of video games out there. The most that would happen would be something on Stern's server end, but it wouldn't be on an individual machine level.
Just think of it this way, Stern's had code to download on their website for 10+ years. At any time, someone could have hacked in and uploaded a bad update for a game. Never happened...because nobody cares. We can't even get complete code a lot of the time, let alone make the machine do something else weird, lol...the games own programmers don't want to work on it anymore, why would a bunch of hackers?