Apple is aware of the issue and will make a fix available in a software update. Until the update is available, iPhone owners can use these steps to re-open the Messages app:
- Ask Siri to "read unread messages."
- Use Siri to reply to the malicious message. After you reply, you'll be able to open Messages again.
- In Messages, swipe left to delete the entire thread. Or tap and hold the malicious message, tap More, and delete the message from the thread.