DIAGNOSIS The jar seal (o-ring gasket) has failed. It's a large black o-ring seated in a groove around the jar's blade assembly.
FIX You need a replacement jar base gasket. Unscrew the blade assembly from the jar using the special wrench or a large socket. Carefully pry out the old, flattened o-ring. Clean the groove. Press the new o-ring into place evenly. Reassemble.
IF THAT DIDN'T WORK The jar itself may have a hairline crack near the base. Inspect it closely under bright light. Cracks require full jar replacement.
HEADS UP Overtightening the blade assembly can distort the seal and cause early failure. Hand-tighten firmly, then give the wrench a small, final nudge. Do not crank it.