Gaining S-OFF on your HTC device is one thing you must do if you really want total control of your device. Once you are S-OFF, it will be much more easier to flash custom ROMs, custom bootscreens, recoveries, etc. For the HTC One M8 (AT&T version), gaining S-OFF is now possible and this is what the guide below helps you to achieve.
I. Before You Begin
1. Make sure you have the proper drivers installed on your PC before starting. Installing HTC Sync will usually take care of this (but be sure that HTC Sync is not running during the procedure; if it is, open up the Task Manager on your PC and end anything related to HTC or adb).
2. Charge your device to 75% or more before you being to prevent unwanted shutdowns.
3. Back up everything you feel is important before you begin.
4. You need to be rooted for this procedure to work. Use our guide here, if you are not already rooted.
II. Necessary Downloads
1. firewater (link available on this page under the Instructions section)
III. Gaining S-OFF on the One M8
1. Extract adb-fastboot.zip to a folder on your desktop.
2. Copy firewater to this extracted folder.
3. Hold Shift and right-click on any empty space in the extracted adb-fastboot folder and click on Open command prompt window here to open up a CMD prompt window.
4. Enable USB debugging on your device by going to Settings > Development options and ticking the USB debugging checkbox.
5. Connect your device to your PC. If there are drivers being installed at this point, let them do so before proceeding. Also, if HTC Sync is running, terminate the program via Task Manager.
6. Run the following commands in the CMD window:
adb wait-for-device push firewater /data/local/tmp
chmod 755 /data/local/tmp/firewater
7. Type in Yes when prompted.
All set! You have now gained S-OFF on your AT&T One M8!