You can mostly rust it because putting a well-hidden backdoor is a lot of work. This work must be paid for somehow. There must be a way to activate the backdoor, either at the mastermind's command, or in certain interesting circumstances. The activation's effect should be painful enough for an adversary to be worth it. Just bricking a few million consumer DVB boxes is likely not it.

Otherwise, read "Trusting trust"; if you are targeted, trusting anything at all is hard.