As many iOS users already know a bug was found by some Reddit users that could allow anyone to crash your iPhone by sending you a specific string of text, that includes characters in Chinese, Marathi and Arabic. When the person who receives the message opens it up their phone will immediately reboot. After the reboot if the user attempts to reopen the Message App it will crash (If it is opened on the list containing the message). The bug is due to an issue in how iOS processes the text in the banner notification and on the lockscreen.
Fix the Apple iMessage Bug
A few ways have been suggested to fix the problem until Apple releases an official fix. You can try sending a message to the original sender by using Siri or by sending a picture through the photo app. This will stop the messages app from force closing. To stop the issue from happening all together, you can disable notification banners from the messages app.
As of a few hours ago Apple knows of the issue and is working on a fix.