[hazov]

I'm only throwing my opinion, I'm not really into security but I spend some time studying networks and low level stuff as a hobby, any specialist will probably point my errors below.

Depends on the device no? Suppose you have a chip with a backdoor, if said chip depends on a OS to operate and you can for example block connections (using a packet filter, either blacklisting the undesirable ones or drop all except the whitelisted ones), then whatever backdoor is there will need an unblocked connection to be activated.

As I see it the majority of hardware backdoors would only be useful if you can have some sort of access to the device, either remotely in a network the device is connected, physically (as far as I understand that's how Stuxnet spread in Iran) or if the user willingly executes some code that exposes the backdoor to you over some routing.