I have not run into this with the Ninja blender. For other brands, I know that this usually means some parts just need to be replaced, such as the blade housing or assembly, or the gasket.
I used Teflon tape and/or Vaseline around the grooves and on the rubber seal for a 20 year old KitchenAid that started leaking after they no longer made parts for it. This worked for a spell until the problem got really bad a few years later, which is why I bought this Ninja.