A bug in the way iOS handles certain unicode text causes the iPhone to crash when receiving a message. The string of text has been making its way around the Internet after it was first discovered on Reddit earlier this afternoon.

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Power
لُلُصّبُلُلصّبُررً ॣ ॣh ॣ ॣ
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If you send the string of text to anyone with an iPhone, the device will respring and reboot. Jailbroken users will be forced in Safe Mode. If the Messages app was opened to the list view, then the Messages app will become useless as opening it will always result in a crash.

Oddly enough, if you have the conversation window open already and receive the text, the phone will not crash; therefore, in order for the crash to work, the user must not have the conversation thread open (the crash works everywhere else including the lock screen, notification center, and banners).

Speculation on Reddit leads many to believe the bug is caused by the way iOS process unicode text, specifically in notification banners. The banner reportedly attempts to present the incoming text and then "gives up" thus causing the crash.

If you're stuck with your Messages app constantly crashing after receiving the text there are a couple of fixes. Most involve sending a message outside of the messages app (through Share sheets or through Siri). Below are two examples of ways to solve the issue:

1) Launch the photos app, send a random photo to the person who sent you the text, then go into iMessage and delete the conversation.

2) Use Siri to send a message to the person who just sent you the string. After Siri confirms and sends the message, open the Messages app and the problem should be resolved.

To prevent anyone from restarting your iPhone through the message again, you can turn off alert/banner message previews, or completely turn off message notification banners (Settings > Notifications > Message).

https://youtu.be/kcifpTa7E1g