Starting with iOS 12, you will have to unlock the iPhone or iPad to connect to the USB accessory, which is because the ' USB Restricted Mode ' protects the iPhone or iPad from hack tools like GrayKey.
Why do you see this warning?
You see this warning due to ' USB Restricted Mode ', an Apple security feature added in iOS 11.4.1 and improved in iOS 12. This feature prevents USB devices from establishing a data connection in when iPhone or iPad lock. Devices connected to the Lightning port can still charge iPhone or iPad however they cannot do anything if the iPhone or iPad locks.
Because hacking tools like GrayKey take advantage of USB connections to crack protected PINs on iPhone or iPad. To prevent this, Apple restricts USB devices to establish data connections when the iPhone or iPad locks. This is the default setting, you can turn it off if you want, but Apple advises you not to do so because Restricted Mode USB mode prevents others from accessing your iPhone or iPad without your permission.
When connecting a USB device to an iPhone or iPad while enabling limited mode, you will see a " USB Accessory " recording " Unlock iPhone to use accessories ' or" Unlock iPad to use accessories ".
How to turn off Restricted Mode USB mode
As it is, you should not turn off this restricted mode USB mode. When connecting to a USB device, simply unlock the phone or tablet with the Touch ID or Face ID. But if this feature annoys you, you can turn it off, especially when using multiple USB devices.
To disable USB Restricted Mode and let it work even if the device is locked, go to Settings> Touch ID & Passcode (or Face ID & Passcode ), enter the PIN code to continue.
In the Allow Access When Locked section , turn on the USB Accessories option so that the device can connect to the iPhone or iPad when locked.
Again, we recommend that you do not enable this feature. It has to protect your personal data from hacking tools being used today.
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