The blueprint details the iPhone's "iBoot" system, which kicks in when a phone switches on. Hackers could potentially use this to craft a way to install malware or surveillance tools on a victim's device, researchers said.
"It's big, but does not directly impact users yet," said Matthew Carr of Insinia Security.
"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," he added.
Jonathan Levin, chief technology officer at software consultancy firm, Technologeeks, told Motherboard that the code hacking represented "the biggest leak in history".
Apple has always kept its software source code under wraps to avoid anyone stealing or hunting for vulnerabilities that might be used to break the security of its products. It runs a bug bounty program, where eagle-eyed security researchers and white hat hackers are encouraged to alert them of potential security holes in its software in exchange for money.
Rewards depend on the severity of the flaw, but its website states that an iBoot system disclosure could be worth $200,000, as it is a central component of its system.
On Wednesday evening Apple issued a take-down notice to GitHub, where the file was posted. The company is yet to comment.
