Hello I want to talk about two computatoinal security blindspots that many people miss and are kind of kept on the down low to an extent. They are super important nonetheless.
The first is firmware, which is essentially the lowest level code in a computer. Stuff like BIOS, drivers, code that talks directly to hardware. Also, stuff like BIOS is the first thing that runs when a computer initially powers on. So firmware is indeed powerful as it is the first code to run on bootup, and it is practically hardware, so it absolutely has control over the OS, and can absolutely hide from the OS.
TL;DR - Things that run early have more potential power, things that run at a low level of abstraction have more potential power.
OK so what? Nothing infects firmware anyway.
First of all there's Stuxnet, which is a payload delivered via USB drive firmware on plug in. Ask the Iranians about it. https://en.wikipedia.org/wiki/Stuxnet
Then there's malware which can totally infect a BIOS, Lighteater. https://betanews.com/2015/03/21/lighteater-malware-attack-places-millions-of-unpatched-bioses-at-risk/
There are definitely more frimware level attacks.
Defending against this: Update your BIOS (be careful not to brick it), use a computer with an open source BIOS, such as Libreboot (https://libreboot.org/), avoid using protocols that do Direct Memory Access and/or use a USB hardware firewall (https://globotron.nz/collections/all) and in all of your tool use only, only free and open auditable code. and hardware if possible.
OK firmware security is pretty terrible but guess what? All Intel CPUs since the late 2000s have a hardware backdoor in them. Called the Intel Active Management Engine, it is a tiny computer full of encrypted, closed source code lurking within. It can access the internet, read and write to system memory and even view the screen buffer.... https://www.fsf.org/blogs/licensing/intel-me-and-why-we-should-get-rid-of-me
We have a few choices to mitigate this very serious flaw. One is not using Intel. However, AMD has similar issues-
Libreboot on AMD:
"AMD Platform Security Processor (PSP)
This is basically AMD’s own version of the Intel Management Engine. It has all of the same basic security and freedom issues, although the implementation is wildly different.
The Platform Security Processor (PSP) is built in on all Family 16h + systems (basically anything post-2013), and controls the main x86 core startup. PSP firmware is cryptographically signed with a strong key similar to the Intel ME. If the PSP firmware is not present, or if the AMD signing key is not present, the x86 cores will not be released from reset, rendering the system inoperable."
Same issues as Management Engine essentially. Read more on AMD at: https://libreboot.org/faq.html#amd
ARM may be slightly better, but probably not (Trustzone) and you would be drastically limiting your hardware choice.
A better option is Libreboot (https://libreboot.org/) which can install free and open firmware on just a few compatibile computers. This is the way to a 99% open source computer right now.
Another pretty good option is Qubes OS with the right hardware. Its like if everything in the OS was sandboxed from each other in its own VM, watched over by a Xen hypervisor. Pretty impressive work. https://www.qubes-os.org/
Another option in the works is RISC V, which is an open hardware computer processor. We are a ways off from seeing this in your desktop case. But it will happen and people are excited.
Another open hardware processor that exists now is Open POWER but it is really not useful to the average user.
I would like to close by encouraging you to never trust anything with a cellular modem as well. The firmware of every cellular baseband modem is closed source, it is written by AT&T or Verizon of some other big surveillance industrial complex. Furthermore, it can read and inject into your phone's RAM. Do not trust anything with a cellular modem ever. There is no secure phone. This is why stacks of latest gen ipod touches with Signal installed are common sight during police raids. https://www.devever.net/~hl/nosecuresmartphone