I'd have said hugely unlikely, but elements of the following story sound terribly plausible ...
http://www.bbc.co.uk/news/technology-32780513
The key line is this one: "If that were true, Roberts might have accessed the plane's systems and data without permission, but perhaps never sent the real live system any commands to mess with the aircraft's journey"
I suspect that's exactly the situation - he never sent any commands, but he could have - and now, others could be looking into it too ...