Obviously read-only access to some sensor is not the same attack surface as having the ability to remotely shut-down the refrigeration.
- The device’s control has a radio transmitter that emits its readings, but no receiver. Listen for those messages to monitor it but there’s not a physical path to reply to it.
- The device’s controller writes a database or message queue. Its code never reads from that source. Monitors listen to those messages.
And many, many similar methods with the same core concept: the item transmits its status but doesn’t have a method to receive replies. You just have to commit to using any one of those thousands of mechanisms.
Read-only is a strong control with lots of solid implementations.
But to address your specific point:
being able to monitor does not ( or at least should not ) automatically equal put it on the web
Obviously it's a question of cost and convenience, and just dumping your appliance on the internet and make it phone home is going to be the cheaper option. If you want security those appliances goes on a separate network, which cannot reach the internet.
This line suggests the units themselves were targeted individually with a hack:
> Danfoss said the affected cold units were older models whose security software could not be updated to ward off a risk they had identified.
But the problem with networked devices spread across tens of thousands of pretty much unguarded locations, surrounded by unvetted employees, but taking orders form a central location is that there is no such thing as "not online". They'll talk to the management server used to set up the nightly defrost schedules across the fleet. Even if they are in an isolated VLAN, it's always one step away from someone connecting a Pi to that network and off they go online. There's only so much zero-trust, NAC/802.1x and segmentation can do when some devices will be too old to support these and their network will be the entry point to the management server, then to the whole fleet.
I'm honestly very surprised that a couple of things things haven't happened yet: the central server for managing these fleets getting hacked and allowing access to the entire fleet, and/or the companies realizing that the only way to limit the reach of an attack is to physically not have the hardware needed to do remote control.
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.
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.