A common problem is plastic leaking around the nozzle.  Different websites and Wikis suggest to use Teflon tape on the threads for sealing.  That should not be needed if the parts are properly made.  The Ultimaker nozzle and brass heater barrel that I received are very well-made.

There are only two locations that can leak:  Each end of the brass barrel.  Threads do not leak, that is a symptom of a failed seal at the barrel tip.

The biggest problem is wrong assembly! The nozzle should be tightened properly against the brass barrel, not against the aluminum block.  Note, the seal is at the tip of the barrel, not in the threads.

Temporarily add a couple of lock-nuts to the barrel to allow sufficient tightening. After removing the lock-nuts, thread the barrel through the heater block and the plastic insulator.  There should be a slight space between the nozzle and the aluminum block to guarantee that the seal is proper between the nozzle tip and the barrel.

If the nozzle is not sealed against the barrel tip, plastic can leak up the threads and if the nozzle is sealed  to the heater block, the plastic can continue up through the heater block and leak on the top of the block.

Also note that if the nozzle afterwards is tightened up against the heater block, the force will microscopically pull out the barrel from the nozzle and possibly create a leak at the barrel tip.

Similarly if the peek is leaking it might be coming from a poor seal between the top of the barrel and the Bowden tube and in that case that joint should be fixed or you might be getting plugs there.

There is also a suggested fix by UM to feed a short length of ABS through the extruder and it will help to seal leaks if only PLA is used..

Note, if the nozzle or hardware is not properly sealing it might be necessary to use a sealant.  It is the old thing: Understand it Before Fixing it.