DIAGNOSIS Liquid leaking from the jar's base means the blade assembly seal has failed or wasn't installed correctly. There's a critical silicone gasket on the blade shaft that keeps everything contained.
FIX Unscrew the blade assembly from the bottom of the jar. You'll see a black or gray silicone O-ring/gasket seated in a groove around the central shaft. Remove it and inspect it for nicks, cracks, or if it's flattened and lost its spring. Clean the groove thoroughly. If the gasket looks good, re-seat it perfectly in its groove—it shouldn't be twisted. Then screw the blade assembly back onto the jar. Hand-tighten it firmly, but don't go crazy with a tool; you can crack the plastic.
IF THAT DIDN'T WORK If it still leaks, the gasket is likely worn out and needs replacement. You can buy a new blade assembly gasket kit. Also, check the jar threads and the blade assembly threads for any hairline cracks you might have missed.
HEADS UP Never run the blender without liquid above the blades. The friction of dry blending can overheat and warp that seal almost instantly, causing future leaks. Also, always hand-wash the blade assembly; the dishwasher's heat can degrade the silicone over time.