Maybe I could be waiting for some specific file to be transferred to the server? Or maybe I could be a creep and monitor all communications in and out?
Look at Equifax, the guys just sat down and held the doors open for themselves for a few months. What if someone did that with the NSA? Valuable data would definitely go through there, and there would be people very interested in getting it, no matter the cost. If that happened to the Pentagon's network... well, that could be really worrisome.
This is'nt exactly how servers are being hacked. Let me give you an example. Wordpress for instance. Used over 40% in the complete internet today. Very populair and hugeeee database to plugins, themes and what more. Complete businesses rely on a simple wordpress website. It's know that wordpress needs alot of updates and esp. on security level, since it is a very bad product from design actually.
When it comes down to security, wordpress could have a zero day exploit, or one of the plugins, which hackers could download, analyse and write a script for it in order to break into. It's really hard to run through every line of code since most people rely on the functionality of the plugin or theme and just expect the developper to do his homework. Anyway, so lets say a plugin or theme gets hacked, it's a fairly simple task to 'upload' a file to the server, and execute it remotely.
It's called a shell. Now a shell is just a little script packed with all sorts of stuff to discover the server, configuration and even extra's to drop another payload. It's very populair these days. I've seen company's install wordpress under ROOT level (lol) which is screaming for trouble's. If a server has outdated or unpatched software it's just minutes work to pass through any flaws. But the main question is will the server itself be hackable in this case in order to get ROOT rights and start updating biosses.
If you get root in the first place, you dont need this specific exploit anymore since you can do whatever you want with the server.
If i'd sell motherboards on Ebay tomorrow with modified biosses, that already poses a security threat since i could program it to make a call to home. I'd know the user, the IP and i could do anything related to it remote, that's what this exploit is basicly about. But if you overwrite the bios with a latest and brand new one, pretty much as good as that the exploit is gone.
They have a point; there's lots you can do with it, but not as critical as Meltdown or Spectre was.