Solid wood newel posts are bolted or screwed to the floor, a stair tread or riser, a stringer, or even a joist underneath the floor. Like the hanger bolt in the railing, these fasteners may be concealed by wood plugs. Remove the plug and tighten the fasteners if possible. If you can't find any fasteners to tighten or if they are inaccessible, drive a new lag screw to secure the loose newel to the stair stringer.

Using a 3/4" spade bit, drill a counterbore into the post, about 3/4" deep. Aim the hole toward the stringer. Drill a 7/32"-dia. pilot hole through the center of the counterbore and into the stringer. Then, widen the pilot hole through the post only, using a 5/16" bit; this prevents the screw from splitting the wood. Fit a washer on a 5/16"-dia x 4"-long lag screw and drive the screw with a ratchet wrench, but don't overtighten it. Plug the counterbore with a glued piece of wood dowel.