Diagnosis
Probably not defective, but the blade isn't sealing correctly against the cup. This is almost always because the blade assembly isn't screwed on tightly enough, or the rubber gasket is damaged or out of place.
How to Fix It
1) Empty the cup and unscrew the blade assembly. 2) Check that the black rubber gasket is seated evenly in its groove all the way around. If it's twisted or pinched, reseat it. 3) Look for any nicks, cuts, or permanent bends in the gasket. If you see damage, you need a replacement gasket (you can order them online). 4) Screw the blade assembly back onto the dry, empty cup. Tighten it firmly by hand until it won't turn easily anymore. Don't use tools, but give it a good, solid twist.
If That Didn't Work
Inspect the plastic rim of the cup itself for hairline cracks, especially near the top threads. Even a tiny crack will cause a leak. Also, check the plastic blade base for cracks around the center post.
Heads Up
Always assemble the cup and blade when both are dry. A wet rim can cause the gasket to slide and not seal properly. And always store them separately, not screwed together, to prevent the gasket from being compressed and losing its shape.