as for the magic code which tells aircraft to switch off. really?......really? and how would that work then? Say, if you chose to operate with no active or passive transmission devices?
Easy enough. You don't even need a trojan horse in the aircraft There are plenty of receivers on the aircraft, code is transmitted by radio. received and processed, and the code can work in many ways:
Virus which reverses control movements
Virus which removes protection from electrical generation systems, causing overheat and probably system shutdown or electrical fire
Virus which kills the FADEC
the list goes on.
The point is all you need is to know how to get around the protection systems (a back door, which almost all software developers create) and you can do what you like.
It may sound far-fetched and Clancy-esque, but it is well within the bounds of current technology.