If they have been hacked then the days of commercial aviation are over.
IF....if indeed terrorist no matter their origin can hack any modern jet was possible I honestly believe it will first off be an all out campaign to knock the rumor down, at any cost, even if it means pulling the plug on the internet. NSA, CIA, any of the agencies will NOT want that rumor to expand.
Seriously, if the public became aware it was even possible there would be a mass panic of flyers, people would refuse to fly unless in a Cessna 150.
Lets assume for a moment it could happen, think of the repercussions to the airline industry, also think of this event as being a “trial run” successful or not, if it was then its already successful.
This all out terrorism, creating fear, fear of attacking Moslems worldwide, because now they have the means and ways to indiscriminately just yank a plane out of the sky, kill all or most on board and then crash it, without even being aboard.