DIAGNOSIS Liquid is seeping past the blade assembly seal. The black rubber gasket (the O-ring) is either worn out, cracked, or wasn't seated correctly the last time you cleaned it.
FIX Unplug the blender and unscrew the blade assembly from the jar. You'll see a thin black rubber ring sitting in a groove. Carefully pop it out. 1) Inspect it for any nicks, cracks, or if it's flattened and lost its spring. 2) Clean the groove in the blade assembly and the rim of the jar where it sits—any food grit here prevents a seal. 3) Press the gasket back into its groove evenly, making sure it's not twisted. Reassemble and hand-tighten the blade assembly firmly, but don't crank it with tools.
IF THAT DIDN'T WORK The blade assembly housing itself could be cracked. Hold it up to the light and look for hairline fractures, especially around the center shaft. A crack means you need a replacement blade assembly.
HEADS UP The gasket is a consumable part. If you blend frequently or use a dishwasher, it will wear out faster. It's a good idea to have a spare on hand.