Apple dropped details of the new security feature in its release notes, and claims that the move is to protect its newer Apple laptops from potentially malicious accessories.
Users will have no ability to switch the function off and will have to approve a USB-C accessory before it can talk to the operating system by dealing with an annoying pop-up.
Sounds fair enough you would think but it appears that Apple is being selective over what accessory’s have to ask permission.
Apple says this doesn't apply to power adapters, standalone displays, and connections to an approved hub -- and devices can still charge even if you don't approve the accessory, which seems to use to be a security flaw in a “defence mechanism.” If you refuse to allow a device to work, and it is charging it is still connected and responding to directions from the unauthorised machine.
Apple says that accessories that are already connected will automatically work when updating to the new macOS software.