Thanks to a bug in the iOS Messages app it's possible to crash your iPhone with a text message. The bug requires a specific string of Unicode characters, and there are some workarounds while we wait for Apple to release a fix.
Apple is working to fix an iOS bug that lets someone crash your iPhone via SMS
The issue showed up on a Reddit thread where the specific character string was posted. Sending the string as a text message will cause your iPhone to quickly crash and reboot when it tries to show a notification for the incoming chat. If Messages is already open, the app will crash when attempting to display the message.
Redditors are reporting they can consistently reproduce the crash on iOS 8.1.2 and newer. Unless you're specifically targeted, say, by a friend who thinks it's funny to crash your iPhone, you won't likely encounter the issue.
If you're worried you might receive texts with the character string, disable Notifications for Messages by tapping Settings > Notifications > Messages on your iPhone. From there, you can disable Allow Notifications. After Apple releases an update to fix the issue, you should be able to safely reenable Messages Notifications.
If you already received a message with the character string, have someone send a new text to the same chat. Alternately, you can use the Share feature in another app like Photos to send a new message to the same chat.
The Reddit discussion suggests the issue stems from the way iOS displays text in Notifications, and the fast crash and reboot cycle is your iPhone's way of preventing a crash that's more difficult to recover from. Regardless of the cause, Apple is working on a fix that hopefully will be available soon.