[g_p]

Yes, this is really hard.

You could get a Solarwinds type situation where the adversary has the signing keys and ability to publish to the website.

You might also find that the vendor ships a library (like libxz) as a part of their invisible or hidden supply chain, that is able to be compromised.

You might find that one of the people working at the company makes a change to the code to enable remote access by the adversary in a targeted collaboration/attack.

The problem isn't that signing key (although I could delve into the lengths you'd need to go to to keep that secret under these threat models) - the problem is what they sign. A signed end release binary or series of packages isn't going to address the software source code itself having something added, or the dependencies of it being compromised.

[echoangle]

Except for the first point, these things aren’t exclusive to remote updates though. I thought we were talking about the challenges of remote updates compared to other methods (like replacing the system or manually updating it with installation media). Supply chain and insiders would be affected that, too.