Google has begun working on more regular Android updates after the 'stagefright' exploit. The latest security patches are integrated into the so-called December security update.
HTC has been working fast to incorporate those fixes within its phones and recently has begun seeding the update on the HTC One A9. As of today the One E9 and One E9+ are also getting the December Security Update.
The first market to get the update is HTC's homeland - Taiwan, but we expect the update to roll out in even more markets very soon.
The new firmware weighs about 360MB and includes all new security fixes, squashed some bugs and improves on the overall performance.
If you own an HTC One E9 or One E9+, you should be getting this update any moment now.