Can a Boeing 777 be hacked?
As of currently, the answer to this question is in principle no for commercial aircraft, at least not remotely. There are two parts to this:

From a system perspective:


Aircraft systems could probably be 'hacked'- assuming you could for instance screw up the flight computer by changing the chips in the belly- but there is no way you could really pull this off without the pilots noticing the course being way off from the compass for instance, so they'd figure out and shut it off. Many systems are not connected in a way that allows them to be cause damage to others. Systems are also pretty dedicated- if it's supposed to fly the plane, that's what it does, if it's supposed to navigate, that's what it does.
Systems on aircraft generally a have limited, if any, connection to the outside world. There are a few systems that have the ability to connect to the ground, and this is generally limited to simple text messages. Most of the electronics on the 777 are from the type's introduction in 1994. A decent analogy I think is trying to hack an early cell phone remotely. 


And from a practical standpoint:


Aircraft are generally very stiff when it comes to changes- they are often approved in a certain configuration, and that then sticks, so there's little need to change stuff, and hence it's not possible. This applies to the autopilot and flight controls- once it's there, it stays there.
As a few people have pointed out, these systems are proprietary and closed source, making it difficult to program them if you're an outsider, but in theory it might be possible.


The fact that no aircraft, even the computer-intensive Airbus, have really been hacked to any extent, the notion that the entire aircraft would be taken over and flown somewhere is impossible.

As for original question link:


  “The FAA has determined that the hacking technique described during a
  recent computer security conference does not pose a flight safety
  concern because it does not work on certified flight hardware,” he
  said.
  
  “The described technique cannot engage or control the aircraft’s
  autopilot system using the FMS or prevent a pilot from overriding the
  autopilot,” said Dorr.
  
  “Therefore, a hacker cannot obtain ‘full control of an aircraft,’ as
  the technology consultant has claimed,” said Dorr.


On a similar accord, I think I heard a story about a Bombardier Dash 8 that got a computer virus once, but I think it never got beyond the navigation system, the most processing intensive part. Unfortunately, I can't find a source for this.