In a development that could allow hackers to find security holes in the product, part of the "source code" - which is the instructions underpinning the iOS software - has been made available on computer programmers website GitHub.
As reported by the Daily Telegraph newspaper, an anonymous user uploaded the code, which relates to iOS 9 - a three-year-old version of the software - although it's claimed that the components will most likely still be there in the latest update.
Despite the leak not having a direct impact at the moment, it has been claimed that the "visibility" it gives into the working of the code could be useful for hackers.
Insinia Security's Matthew Carr said: "It's big, but does not directly impact users yet. But it gives visibility into what the code does so anyone looking to reverse engineer iOS and write exploits can use this to make their job much easier.
"There may even be massive parts of code reused so they could try and find bugs in old code and see if it works on new versions."
Jonathan Levin, the chief technology officer for Technologeeks, has suggested to Motherboard that it could be the "biggest leak in history".
The leak hasn't put owners at risk immediately, but there is the possibility the code can be replicated and manipulated with malicious intent, with users potentially vulnerable in the future.