How to fix loose box newel handrail post
To remove the post, I had to remove the trim piece under the handrail. It was held in with very fine brad nails and I had to wedge a putty knife in there to get it loose. That allowed me to remove the handrail from the post.
Once the post was free, I was then able to slide the post out. It was fastened with the keylock type post fastener. The fastener was screwed into the tread and 2x4 underneath and the screws had come loose. I tightened them, but I suspect they'll just come loose again.
I think the long term fix would be to use long bolts with a plate underneath to keep them tight.