Security expert Justin Case of TheRootNinja has posted a video to his youtube channel proving that he has rooted the HTC One A9. The root method consists of applying a modified boot.img. You should also already be bootloader unlocked via HTCdev.com. You will need the SDK installed to your PC or at the least have proper fastboot files and adb files on your PC.
Once you have all the prerequisites you will need to flash the boot.img, factory reset your phone, flash a custom recovery, boot into recovery, then flash the supersu.zip. Then you are finished! Head to the link below for the full guide and file downloads.
via XDA