Originally Posted by GSLEON3
Take it to AT&T first, but if it is after 30 days, they will likely make you deal with HTC.
They changed the in-store return policy some time ago. You only have 14 days to swap in-store for a new device. After that, you go through AT&T warranty department and will probably get a refurbished M8.
But you don't ever have to deal directly with HTC, if you purchased from AT&T.
I assume OP has checked the glass on both cameras to make sure its not just some dirt or dust on the surface of the glass? I've also noticed a spec of dust under the glass ("inside" the phone) but in front of the actual camera module (top camera), on my M8. It seems to be out of the way, and can't be seen in photos. Although its possible it will move in the future, and be seen in pics. Can you see anything like that inside? Not something you can fix easily. But at least might give you an idea what is going on.
Otherwise, maybe it is a defect of the camera sensor itself, as the OP mentioned.
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Phone: AT&T HTC One (M8)
Tablet: Samsung Galaxy Tab Pro 8.4 (WiFi only)