Confirmed working first by "italyforever" - I could not test myself since I sold my M8, just wanted to help others out. Confirmed working by several other members too.
At&t 2.23.502.3 Ruu has just come out and I have had requests for bootable RUU. Here it is first from me! Place this file in the root of your ext sd card and boot to hboot and hit vol up to update. If you are soff you do not have to lock boot loader. If you are s-on you must lock boot loader first. Please follow my guide for the 1.58 bootable RUU for any questions. It explains everything. Do not try to modify file as it is singed and encrypted by HTC and will cause issues if you mess with it. If you try to unzip it, it will say corrupt because this is an encrypted file. Just verify md5 to make sure your file is good. The restore takes 14-17 minutes. Plugging in phone is a good idea so there is no chance of battery dying. It will do a bunch of different unzipping and installing. It will bypass certain things too, this is normal. Let the update finish. It will say update complete press button to reboot. Do not touch phone during this process! This is ONLY for the at&t M8, no other variant. CWS__001 or 11111111 cid required. This will erase your phone like new so backup anything needed. I am NOT responsible for anything that happens! Please click my thanks button if I helped you.
2.23.502.3 bootable RUU - https://www.androidfilehost.com/?fid=23622183712463537
Old guide - http://forum.xda-developers.com/att-...hboot-t2818556