alt Hacker NewsSo this is how it was done 25 years ago: Only the used ports (ethernet jacks) on a network is activate, disconnecting a device and the port closes. That strictly speaking doesn't mean that there aren't way to get devices on the network, but it does make it a lot harder. You're absolutely correct about these networks being in hostile environments, but why then do we pretend that they are not? Fridges like these have never not been in a hostile environment, so why aren't basic security measures taken to secure the physical side of things?
In my world the central server can be hack in two ways, that is by a malicious operator or via some scheme like the US/Israeli attack on Irans uranium centrifuges. The central server also have no business being online, that to is on a closed network, running the whitelisted applications from the manufacturer.
This is how it was done in this [0] very nice 60 years old (I'm sad now) movie. [Spoiler alert] An alarm is triggered so many times the guards just disable it. Alert fatigue has worked the same forever. Nobody wants to check the thousands of false alarms that probably happen every day across thousands of locations.
As long a schedule is conceived in one place and applied in another, even with human intervention on both sides, either one of the sides can be swindled. At this point with the bar for a successful hack being lower than ever, completely removing the hardware necessary to make remote changes is probably the only way to limit the blast radius.
Maybe allowing to send the schedule but requiring a local employee to operate a local hardware only control to apply it could work, but who has the money for this kind of complexity. Something like this on every freezer (device, really) in a fleet probably costs multiples of the money they lost with the spoiled food.
[0] https://www.imdb.com/title/tt0060522