Once the iPhone is plugged, the hacker bombards the device with different combinations of the passcode.
In the article, Apple mentioned that iOS is a secure operating system with many inbuilt security features to protect users against malware and viruses.
But Hickey was able to make multiple incorrect attempts to unlock an iPhone until finally guessing the right passcode (which he knew already). This is a problem faced by law enforcement agencies when they encounter iPhones in the cases they're working on - as well as people trying to hack into phones for nefarious purposes - so it's little wonder that hackers are constantly trying to find a way to earn unlimited guesses at passcodes. So while it looks like the pins are getting tested one after the other, the codes aren't always sent.
And now Apple has stepped into the fray and dismissed Hickey's initial claim in a brusqe statement.
Going back to his research, Hickey found that while it appeared that 20 or more passcodes were being entered in his brute force attempts, the iPhone was only recognizing four of five passcodes. By "modding" the iOS, you can download applications from third-party websites and stores, customize the interface of the device, etc.
Hickey claimed on Twitter that he had a way to "brute force 4/6digit PIN's without limits". Apple is introducing a new USB Restricted Mode with iOS 12 that will prevent use of the iPhone's USB connection if your iPhone hasn't been unlocked in the past hour. And while this method will also work with six-digit passcodes by running all of the possibilities between 000000 and 999999 at one time, it would take weeks for the iPhone to complete the task. It didn't say what exactly was wrong with it, but it's possible it noticed the same limitation which Hickey found out about afterward.