International standards can also be zero days. For example, the international standard for called ID, uses a dial up modem protocol which means the hardware has to have capabilities to read this data which means the hardware has dialup modem capabilities. So if you can compromise the firmware of the modem, a backdoor into a device is via the very telephone network you later rely on to communicate. https://en.wikipedia.org/wiki/Caller_ID#Operation

Chips fall into 3 categories, fused by manufacturer, fused by branding company, or not fused allowing future updates, like bios chips. Even if the chip is fused, a backdoor may still exist, in some cases standard behaviour can be the backdoor.

With zero days appearing in hardware, software and standards, its very easy if you have the knowledge to get a persistent backdoor into a device beit a PinePhone, Librem 5 or Necunos to name just a couple. https://forums.puri.sm/t/comparing-specs-of-upcoming-linux-p... https://tuxphones.com/yet-another-librem-5-and-pinephone-lin... Its probably why people like Cobham, Thales and other openly public military manufacturers make their own kits for militaries around the world. https://www.cobham.com/ https://www.thalesgroup.com/en

I even think its possible to hack the communication systems of the new Lockheed Martin F35, because the manufacturers are walking a logical development path (their mistake), but I've yet to have a go at it, so cant say for sure yet. https://www.youtube.com/watch?v=_C25CwNlVjA