Oleg Afonin of the desktop and mobile forensic tools maker Elcomsoft wrote that though the feature had not surfaced in the 11.4 final build, it had appeared in the 11.4.1 beta.
He said USB Restricted Mode had been described as follows in the 11.3 beta: "To improve security, for a locked iOS device to communicate with USB accessories you must connect an accessory via lightning connector to the device while unlocked — or enter your device passcode while connected — at least once a week.”
Focus on the iPhone's security has grown after the FBI had a stoush with Apple in 2016 over gaining access to an iPhone used by a terrorist to kill people in San Bernardino, California.
This feature seemed to be aimed at tools such as those made by Cellebrite and GrayShift that could crack passcodes, Afonin said.
He pointed out that among the options in the "allow access when locked" dialog, Afonin provided a screenshot (above) that showed one titled "USB Accessories".
"Once the user toggles the 'USB Accessories' switch, the iPhone will require you to 'Unlock iPhone to use accessories' to connect (seen on the left) when it has been more than an hour since your iPhone was locked," he wrote.
Afonin said it was clear this was the “proper” USB Restricted Mode because, unlike before, there was zero data communicated over the USB port once the feature kicked in.
"iTunes does not see the device at all; no 'unlock this device to access' and no pairing request. The iPhone just charges off the computer’s USB port, transmitting no information.
"We have not been able to access even the basic information about the device using the Elcomsoft iOS Forensic Toolkit I(nfo) command, the very same command that returns identification information about an iOS device even if it has never been paired with the computer."
However, Afonin noted that there may be a possible workaround. "As was the case in iOS 11.3 beta, the clock starts ticking after the device is locked or after the device is disconnected from a trusted (paired) computer or USB accessory (we were able to positively verify the latter by running a simple test).
"In order to keep the USB port unlocked, the police would have to connect the iPhone to a trusted device during the first one hour, and keep it connected at all times before they have a chance to attempt acquisition."
Screenshots: courtesy Elcomsoft