There’s an iOS bug that lets someone crash your iPhone by sending a text message from their own iPhone containing what appears to be a single line of seemingly innocuous Arabic script, according to AppleInsider (http://tinyurl.com/oa58zch).
When a text or iMessage (but not part of a normal Messages conversation) containing the text string is received and presented in the lock screen or as a drop-down notification, iOS consumes resources trying to render the message, ultimately causing a crash. Some suggest turning off Messages previews for Notifications, though AppleInsider says its tests haven’t shown this to be effective. If Messages has already crashed, however, AppleInsider has confirmed that receiving a new message, sending out a picture from Photos or sending yourself a message through Share Sheets will stop the process.