Matthew says that there's a hack that will crash the iPhone through iMessages. It'll crash the Apple Watch as well. Leo says it overloads the buffer overload and dumps the text on the memory, crashing the phone. And that's how serious exploits could occur as well.
But there is a temporary fix from Apple:
1. Ask Siri to "read unread messages."
2. Use Siri to reply to the malicious message. After you reply, you'll be able to open Messages again.
3. If the issue continues, tap and hold the malicious message, tap More, and delete the message from the thread.
This will prevent the message from crashing the iPhone and iMessage app until they fix it. The downside is, this doesn't do anything to prevent people from sending you this message again. Turning off notifications may help stop it, though. iMore has an article on how to minimize the effect of this bug.