In HTC’s first, the Sprint One M8 is receiving the Stagefright bug fix update, with software version 4.25.651.18. There is nothing else of note included with the update it seems — or at least nothing that Sprint thought necessary to mention in changelog. Btw, the One M8 Harman/Kardon edition is also receiving the same update, while obviously, the software version remains exactly same.
To get the Sprint HTC One M8 update that fixes Stagefright bug, pick up your device now and go to Settings > About device > Software update. Tap on ‘check for update’ button now. You should be receiving the update in next few days under the gradual rollout. If you ever installed CWM/TWRP recovery, you’ll need to revert back to stock recovery to be able to install the OTA update.