Lighting, Light Fixtures, Ceiling and Exhaust Fans - Browned wiring in light fixture - root cause?

While cleaning out dead flies out of a number of hanging light fixtures, I noticed that two separate fixtures in the upstairs hallway had browned/singed/darkened white wiring from the socket up through the penetration into the box. Each fixture has two sockets, each rated for 60W, and are on the same two-way-switch circuit. The fixture enclosures are darkened glass and fit fairly close such that there is not a lot of room at the top for air circulation but they are not totally contained. The white wires are labelled as 18 AWG 600V 150C. The home was built in 2008.

The white wires are most darkened where they penetrate up into the box through a small hole, which is where the white wire runs together with the black wire up throught a 1/8" (approx) hole up into the box. The most browning is at the penetration point and extends about 1 to 2 inches, decreasing in severity the further away from the penetration. Of the two fixtures affected, one had both bulbs burned out at approximately the same time, and has more extensive browning of the white wires than the other fixture.

My first thought was overlamping but the bulbs were all 60W. Any other thoughts that would explain this? Should I replace the fixtures in question? Many thanks in advance.

The metal fixture base is acting like a heat sink and absorbing the heat from the bulbs. It is transferring some of the heat through a very small contact area as the wires pass out to the junction box.

The wiring you listed was a highter temperature rated insulation which should help. I would monitor the situation to see how this is progressing.