Leaking along Chimney

I explored our attic for a new lighting project that I am doing and what I noticed is the wall (right under roof) next to Chimney is leaking bad. The wood is wet and a small section is rotten. There are holes through which I can see daylight. Fortunately, the studs look good.

Later noticed that the wall above fireplace is a bit damp ... it was covered with a mirror.

I guess the correct solution is to install vertical flashing but do we need to remove the siding before that. It is a pain if we need to remove the siding ... it is vinyl siding.

For now, I used some backer rod and foam spray from inside to plug the holes . don't know if this will help .

As for flashing, any tips how to install it ? Do we nail it to the chimney and siding ? This will be first time for me.

 

There is no way to stop it from the inside.
Post some pictures of the chimney from up on the roof on the outside.

 

Thanks for the quick reply. I will need to wait till spring to go up the ladder ... outside is snow and ice covered now ... not safe for a rookie like me