The message in question has to contain a single character from the Indian Telugu language (not included here for obvious reasons). When the message is opened on an iPhone running iOS 11.2.5, the iPhone's native Messages app and certain third-party app including WhatsApp and Facebook Messenger will crash. In some cases, the iPhone will simply revert to its login screen and require you to enter your PIN again.
Apparently the only way to fix it is to delete the message thread, which you'll presumably have to do from a Mac app or desktop web browser, depending on the service. If that's not available to you, you might be able to delete the thread when you receive a new message on it.
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