If split then yes it needs replacing though a good wrap with waterproof duct tape or bitumastic caulk will keep it intact and dry until a roofer can fix it.
Replacing roof vent pipe boot.
Vent pipe rubber boot repair step 1.
Use common sense safety precautions and make sure you can.
Video turorial on how to change out a plumbing vent boot.
You can also replace the entire flashing around the pipe.
I purchased a plastic flashing assembly from the local home improvement store.
Easy installation and no tools required.
However replacing the entire flashing can be quite involved and may require the application of new shingles.
There are a number of different methods for repairing a roof boot.
Now it is time to work on the roof.
Obtain another rubber seal.
Make sure you need a new boot may just need a bit of bitumastic caulk if it is leaking a bit at the duct contact as they commonly do as the rubber gets older.
Slides over the existing roof vent boot preventing future leaks.
A gasket less two piece pipe boot repair system designed to repair the leak around the vent pipes that penetrate the roof.