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You are aware these are freezers, right? Like they're important but they're not holding onto private data or whatever.
Emerson writes the password on the device for technicians to have access to, and Ke2therm uses all fives(no, it can't be changed).
The IT-world argument is that once you compromise any system on a subnet, you can at least listen to everything else on the subnet, and often attack everything else, too. While it's not entirely true in a modern world, it's pretty close. Conficker is the historical example of a wakeup call here. I get that HVAC techs don't really think that way (and, hell, some IT people don't, a la the 'air-gapped' networks that are really just bad firewall configs), though.
The HVAC argument is that a freezer failure is easily five digit cost of damage, even in the optimistic case where the food is just unsaleable rather than inedible, I can't imagine an expert working that job hasn't seen it. The defrost attack here might not be foreseeable for the average HVAC tech, who might reasonably expect it to 'just have a max timer', but in turn I don't think it's the only or even worst option. I don't have a good model of industrial freezers, but from what happens to consumer air conditioners or freezers when they're stuck short-cycling, I'm guessing that would be Bad, and the sort of failure that requires long technician hours and longer lead times on parts.
The former's actually pretty reasonable, at least if it isn't something stupid like 'password', and NIST recommends it for a lot of devices initial configuration. The latter, I'd like to just have a talk. Holy shit. That's literally worse than the 'some idiot has on his luggage' option.
A stuck in defrost wouldn't necessarily cause short cycling(although depending on the control setup there might be some). It would just keep the freezer off. They could replace the controllers with mechanical versions pretty quickly if they can get skilled labor onsite in time, but they wouldn't expect to have a controller programmed to stay in defrost indefinitely- no one would do that on purpose.
Ah, sorry, I was thinking short-cycling as a separate failure mode. Naively, I'd expect an MCU reboot loop to poll the thermistor(s), if they're over target temp engage the compressor, trip a safety or watchdog five to thirty seconds in, and then restart. An attacker that wanted to intentionally trigger that behavior, rather than 'just' triggering an indefinite defrost, should have been on the mind of whoever set up the mainboard's program and most HVAC techs, and...
... Uh, long-term effects I'm not describing here, but unless the answer is 'oh I could clean that out in an hour', I'm not sure should be described here.
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Well, about that...
Yes, running defrost heaters and the compressor at the same time is very bad for the system, like sugar in the gas tank level bad. But that's not what sticking the system in defrost does; it normally prevents the compressor from running at all. That guy bypassed stuff manually which in his case blew up the system(normally it just tanks the compressor).
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