DIAGNOSIS: The silicone seal (gasket) under the blade assembly has hardened, cracked, or was improperly installed after cleaning.
FIX: You need to replace the jar base gasket. Unplug the unit. The blade assembly is removable. Unscrew the retaining nut from the bottom of the jar (use a wrench, it's on tight). Carefully remove the blade, washer, and old gasket. Clean the seating area thoroughly. Install the new OEM gasket (part # KCB96NW or equivalent for your model), reassemble, and tighten firmly. Do not over-torque.
IF THAT DIDN'T WORK: If leaking persists with a new gasket, the glass jar itself may have a hairline crack near the blade stem. Shine a bright flashlight inside the jar while it's dry and check for cracks. A cracked jar is not repairable and must be replaced.
HEADS UP: Never run the blender without liquid. Dry blending or with minimal liquid creates suction that can pull the seal out of position and accelerate wear. Always inspect the gasket when you clean the blade assembly.