As reported by Motherboard, certain "key" iOS source code was recently published on Github for the world to see. While Apple later filed a copyright takedown request which forced Github to take the code in question down, the harm may well have already been done.
The code, labeled "iBoot," is responsible for loading up iOS and verifying the kernel is "properly signed" by Apple. As noted by Motherboard, this code is essentially an iOS device's BIOS.
"This is the biggest leak in history," iOS device internals expert Jonathan Levin told the outlet, adding that it's a "huge deal" for Apple's users. With the code out in the world, hackers could probe it for vulnerabilities and find ways to exploit the system to the detriment of the greater iOS user base. On the other hand, researchers may also choose to poke about in the iBoot code, potentially disclosing any vulnerabilities they find to Apple.
Still, it's worth noting that this leaked source code was reportedly specific to iOS 9. While some code has almost certainly been reused in iOS updates released since then, it might be harder for hackers to take advantage of now.