Toilets, Sinks, Showers, Tubs and Disposals - Vent Pipes

I noticed that the ceiling in my upstairs bathroom had a leak mark that appeared to be leaving a black water mark where the ceiling and wall meet. I went into the attic and found the source of the leak to be around where the vent pipe went out the roof. I could see where the fluid ran down the pipe at a point where two pipes are connected - the one coming thru the ceiling and the one thru the roof. I couldn't see any water run stain on the pipe that goes thru the roof. Could there be water in this pipe that can be coming out the joint? If so what could cause water in these pipes? How could I correct the leak? It doesn't appear the roof is leaking. Haven't had any rain and the spot is wet at the base of the pipe in the attic. Any hepl would be appreciated.

I would guess that someone needs to go up on the roof - the junction where the pipe and the "cap" that fits over it is leaking.

It needs to be sealed or replaced if cracked.

It prabably only leaks on a heavy rain.