iOS Beta 8.4, which is available to developers, fixes the iMessage exploit that can crash the iOS Messages app once it receives specific string of Unicode characters via iMessage or SMS. Sending the string of characters to an iPhone, iPad or iPod touch results in an immediate respring, causing the device to crash and quickly reboot. Apple issued a temporary workaround a few weeks ago but with iOS Beta 8.4 solves the exploit completely.
Source: Mac Rumors