The bug, which was discovered yesterday, crashes Messages app and in some instances reboots the device after a specific set of unicode string characters are received on the recipient’s device.
Apple is aware of an iMessage issue caused by a specific series of unicode characters and we will make a fix available in a software update. Until the update is available, you can use these steps to re-open the Messages app.
Apple had already confirmed in a separate statement earlier that they are working on a permanent fix which will be rolled out soon, likely next week as part of iOS 8.4 release.
In the meantime, if you are affected by this bug, this is what Apple officially suggests as temporary fix:
Step 1: Tap and hold your Home Button to launch Siri.
Step 2: Ask Siri to “read unread messages.”
Step 3: Now use Siri again to reply to the message’s sender. Once Siri has successfully sent the reply, you will be able to open the Messages app again.
Step 4: Launch Messages app.
Step 5: Swipe left to right to delete the entire thread. Alternatively, you can also just delete that particular message by tapping and holding on the message in question, tap More, and then delete the particular message from the thread.