Normally all security critical systems are kept off the public internet, so there would be no way for a spyware worm to communicate with a back door. However there are tricks like putting the worm on a thumb drive, labeling it porn, and “losing” it in the parking lot of a secure facility.
One wonders if these chips can be queried through their power supplies via the power grid. Doubt it, but it would be interesting. If you did the transmission slow enough, the signal to noise might be lower and yet harder to detect.