Revealed by EverythingApplePro on YouTube, this particular bug overwhelms the Messages app with a string of thousands of special characters that are hidden and tell the receiving app whether the text reads from left-to-right or right-to-left, for example.

When such a message hits your device, it immediately locks up the Messages app.

http://www.idownloadblog.com/2018/05/09/another-unicode-text-bomb-hits-messages-app/