12/13/2019
A bug in AirDrop lets people lock up your iPhone. If you're running an older version of iOS, go to Settings->General->AirDrop and make sure you can only receive files from your contacts ("Contacts Only" setting.)
If you're already running iOS 13, update to iOS 13.3, which will fix the bug as well.
More info:
Apple fixed the bug in iOS 13.3.