Cyber hackers have already shown what damage they can do, but this may be the scariest hacking story yet, if it came true.

What if they could hack into the air traffic control system? It's frightening to imagine, you're at 30-thousand feet and suddenly your pilot is flying blind or taking evasive action to avoid another plane.

Brad "Renderman" Haines, who works in IT support for a small company in Canada by day and is a hacker by night, told CNBC that he has proof that the air traffic control system can be hacked.

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The Federal Aviation Administration's next generation air traffic control system, which, when it's fully in place by the end of this decade, is supposed to make flying "more convenient and dependable."

According to Haines, what it really does is make the whole system vulnerable to hackers because the system uses GPS (Global Positioning System) data instead of just radar.

"No good is gonna come of this," he added.

The problem with this is that the signals they are using are unencrypted and unauthenticated. And because it's unauthenticated, you can't prove that this actually came from the real plane.

In theory, that means a hacker could wreak havoc on the system by generating flights that don't exist.

"You could theoretically ground everybody," Haines said.

Haines and a fellow hacker put data that would be used in the FAA system, known as NextGen, into a flight simulator to show what could happen if the system is hacked.

"We were able to create a flight…we were able to take off from SFO [San Francisco] circle back over the bay, come back and buzz the tower," Haines explained.

The consequence of putting a ghost flight on the air traffic controller's panel could be disastrous.

"If I suddenly injected 50 extra flights onto their radar screen that they hadn't expected, they're going to be panicking trying to figure out what's going on," he said.

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