DIAGNOSIS The silicone gasket on the underside of the carafe lid is worn, dirty, or misaligned. This breaks the seal.
FIX Remove the lid from the carafe. Pop out the circular black silicone gasket. Wash the gasket and the groove it sits in with warm, soapy water. Ensure no coffee grounds are lodged underneath. Dry completely. Press the gasket firmly back into its groove, ensuring it is seated evenly all around.
IF THAT DIDN'T WORK Inspect the gasket for cracks, tears, or permanent deformation. If damaged, it must be replaced. Part number is often listed as a 'lid seal' or 'carafe seal'.
HEADS UP Do not overtighten the lid when placing it on the carafe. It should click into place with moderate pressure. Forcing it can warp the plastic or displace the gasket.