From the person/hacker/security researcher (@axi0mX) who discovered it:
During iOS 12 betas in summer 2018, Apple patched a critical use-after-free vulnerability in iBoot USB code. This vulnerability can only be triggered over USB and requires physical access. It cannot be exploited remotely. I am sure many researchers have seen that patch. That's how I discovered it. It is likely at least a couple other researchers were able to exploit this vulnerability after discovering the patch. The patch is easy to find, but the vulnerability is not trivial to exploit on most devices.
> why make it public?
A bootrom exploit for older devices makes iOS better for everyone. Jailbreakers and tweak developers will be able to jailbreak their phones on latest version, and they will not need to stay on older iOS versions waiting for a jailbreak. They will be safer. It will also be better for security researchers interested in Apple's Bug Bounty. They will not need to keep vulnerabilities on hand so that they have access they need for their research. More vulnerabilities might get reported to Apple right away.
Source: https://mobile.twitter.com/axi0mX/status/1177542201670168576...
I wonder if this is this vulnerability that a private company was exploiting for tools they provided to various law enforcement agencies?