Have you ever had an external canister filter that leaked, specifically where the pump-head connects to the canister body? Most of the time, that’s a seal issue. And most of the time it’s because a seal has gone bad (i.e., the gasket has failed).
However, before you run out and order a new gasket or re-lube the existing gasket, look closely at the top of the canister body. Remove the pump-head and run your finger around the top edge of the canister body. Sometimes, there can be a nic or a little chip in the plastic around the top edge. Sometimes even the molding can have an uneven spot around the top, which might be the problem if your canister leaks right out of the box.
If you feel something when you run your finger around the edge, get a piece of medium/fine grit sandpaper (150-180 grit size) and sand the trouble spot lightly. Just sanding out a small imperfection can be a simple, inexpensive solution. I’ve done it and stopped canister leaks this way.