It’s also the latest move in a cat-and-mouse game between law enforcement and Apple.

Because of the lack of public scrutiny of the feature as well as its covert behavior, defense attorneys, forensic experts and civil liberties advocates are concerned that Hide UI could be used without giving owners the due process of law, such as a warrant. Those NDAs have helped keep Hide UI a secret.

The spyware, a term for software that surreptitiously tracks users, has been available for about a year but this is the first time details of its existence have been reported, in part because of the non-disclosure agreements police departments sign when they buy a device from Grayshift known as GrayKey. Software called Hide UI, created by Grayshift, a company that makes iPhone-cracking devices for law enforcement, can track a suspect's passcode when it's entered into a phone, according to two people in law enforcement, who asked not to be named out of fear of violating non-disclosure agreements. It just has to log the code as the user types it in. But another tool, previously unknown to the public, doesn't have to crack the code that people use to unlock their phones.

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