AT&T has started pushing Stagefright patches to their models of the HTC One M8 and HTC One M9. The patches are rather small and quick, and likely don’t come with any other changes, but they are important. This’ll help shield you against any potential attacks which may cause an attacker to take full control of your phone. An attacker doesn’t even need you to do anything in order for that to happen, so you’d be foolish not to accept the fix.
Unfortunately we’re not yet sure whether this is the initial patch which was found to be a partial fix, or the full fix that addresses every known Stagefright-related vulnerability. You can check for yourself by downloading Zimperium’s Stagefright Detector after pulling the update down. Be sure you’re on WiFi with a charged battery before grabbing it from their servers.