What has been described as the ‘biggest leak in history’ has raised fears that almost any iPhone might be vulnerable to hackers.

It follows a skilled hacker leaking the source code for Apple's ultra-secret iBoot software on Wednesday.

The Daily Mail said the Source code for iBoot, which is the first app that runs when you turn on an iOS device, was posted by an anonymous user 'Zioshiba' on GitHub.

The iBoot app runs when the iPhone transitions from a black screen to a white screen and then the iOS home screen.

The leak alerted various security experts, including one who told Motherboard that it was the 'biggest leak in Apple's history.'

However, the Mail said Apple had since downplayed the leak, saying that the source code was for iBoot running on iOS 9.

Only seven per cent of users are still running devices with software older than iOS 10, which was released in 2016.

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People are advised that if they are using software that is older than iOS 9, which was released in 2015, they should update their device.

The Mail said security researchers are still cautioning that the outdated code could give hackers an inside look into how Apple's secret boot software works.