There are some great presentations on YouTube from the Chaos Communication Congress and similar conferences about the abyssmal security of PLC-controlled indust

There are some great presentations on YouTube from the Chaos Communication Congress and similar conferences about the abyssmal security of PLC-controlled industry stuff, including prisons. It is exactly as you say. They think their systems are too obscure for anyone to figure out. Of course, they hire the cheapest talent they can find to bang the things together in the first place and constantly look for ways to cut staff and replace developers and security personnel with newer (cheaper) people all the time, so they never end up with anything of any significant complexity. Even many places that think their networks are airgapped turn out not to be. In one prison discussed during one talk I watched, the prison had a commissary or something run by McDonald's... someone had connected to the commissaries Internet connection giving the Internet a direct route into all the insecure systems controlling the prison.