You are driving your car when suddenly you discover you are no longer in control. Somebody has hacked into your vehicle's on-board computer and taken over.

If this sounds like a Hollywood plot or a Top Gear stunt, you are wrong. It can actually be done and Sydney hacker Jonathan Brossard knows how to do it.

Vulnerability: ''White hat'' hacker Jonathan Brossard. Credit:James Brickwood

The threat from hackers has been in the news recently, with eBay telling 145 million subscribers to change their passwords after a cyber attack and reports of Apple devices being hijacked last week.

Mr Brossard is a ''white hat'' hacker, one of the public defenders whose job is to preserve the integrity of information systems from the attention of the ''black hats''.