brb reviewing some EE material...
But don't expect much from me... I wanted to do a final year project in Power Line Communication for my electrical engineering undergrad. Then-me decided it was a dead end... cannot go WAN-scale; too much line noise and signal transmission power loss. I did something else with power electronics---I can't remember---but then again, my undergrad was also a dead end :p
I wonder if even "just" throttling up/down the CPU / GPU on tasks as morse code might be visible externally to other devices on the same mains supply. Obviously it would be very inefficient speed-wise, but a higher current (say + 300 watts for the GPU being used) should result in a slightly lower voltage on the mains supply I think for the duration?
Obviously things like PSUs could (I'm not an expert at any of this) get in the way of this, and I'd assume in data centres there's some isolation between different racks.
But for a scenario of DIY "airgapping" two machines in an office for AI testing (i.e. not doing it very well), it might be a plausible way of AIs finding to communicate if there's no actual network connection.
Obviously other electronic devices connected to the same mains circuit could also be adding their own noise, which might make it a lot harder to actually identify and decode the response, but... if ML/AI agents are good at anything, it's finding patterns, so I wouldn't want to bet against it being theoretically possible...
Obviously, given variation in these things anyway (i.e. due to temperature, and PSUs being in the way), it's incredibly unlikely to be that easy to pin down a robust communications method, but again, might be just within the bounds of possibility.
Still, I'd guess that the odds of remote exploits are better if one wants to control a device via the PSU-as-backdoor, versus exfil data from it. Do like the IoT people and UDP broadcast naughty control signals to the power circuit / subsystem that has the compromised device.
Guessing further, remote exfil will mean having a close-proximity wiretap into the precise power line circuit of the target infra---because of signal losses---it could still be UDP broadcast exfil. Or a physical tap at whatever power transformer is the single point of connect between the target and the main grid. And if you can't get anywhere near the installation, well you can't exfil, because AFAIK it's pretty hard to reverse a signal up a transformer back into the main grid.
A scan of the arxiv paper appears to suggest similarly---the situation they describe presumes a wiretap into the circuit or the mains switchboard.
Paraphrasing:
The only way to secure a computer is to not buy one. And if you buy one, to never plug it in. And if you plug it in, to never switch it on.
Something like the cheapest way to own $GasGuzzlerWithItsOwnPinCode is to bring it home after purchase, and set fire to it in the yard.
Some of the sites I look after at work still have modems attached to the incoming electricity meter from a Broadband-over-Power-Line trial about 15 years ago. It wasn't very fast - it was supposed to be 200Mbps but struggled to do 20, and was often more like 2Mbps - and it radiated RF hash everywhere powerlines ran, from about 500kHz to well up into the hundreds of MHz. What eventually killed it in NE Scotland here was it splattered all over marine VHF, in an area with lots of marine traffic like oil platform supply vessels.
TL;DR anything that uses basically RF over unshielded untwisted cables that radiate everywhere is inefficient and antisocial.
Still available today, and has worked well for me when I've needed it.