Apple posts official workaround for Messages bug, says it will be sending a fix
Apple has updated its support pages to include a workaround that you can use until a fix is sent out by the company. The workaround has three steps. One, ask Siri to read unread messages. Two, use Siri to reply to the malicious message. Once you reply, you will be able to open Messages again. Lastly, go to Messages and swipe left to delete the entire thread. Or, you can long press on the malicious message, click on "More" and select delete.
Apple says that a permanent fix is being worked on and will be included in a future software update. No date or timetable was mentioned as to when Apple iPhone users can expect to receive this update. Yesterday, we passed along some unofficial ways to workaround the issue.
source: Apple via AppleInsider