If your sink is leaking underneath the sink in the cabinet it might be a leaky pipe. More likely it is a leaking sink drain. In the case of a sink drain leak, the flange is loose or misaligned and a small amount of water seeps between the sink surface and the drain/drain pipe connection.
The job of a sink flange is to connect and maintain a seal between the drain pipe, and sink surface so running water goes down the pipe and not onto the floor below. In the case when a garbage disposal is installed, the flange helps maintain a seal between the top of the disposal and the sink drain as well.
Over time the flange can move slightly either due to shaking from the garbage disposal, movement of the sink, or simple wearing down of the plumbers putty that holds the sink drain flange in place. Cracked or dried out gaskets or plumbers putty can cause small leaks around the drain pipe as well. Left unchecked, the leak can grow and cause water damage or even electrical damage if there is wiring under the sink. Fortunately it is usually pretty easy to fix a leak in a kitchen sink flange.
To begin you will want to turn off the garbage disposal (by shutting off the circuit breaker on your breaker box for the disposal).
Use an adjustable wrench to loosen up the drainpipe. You can do this by grasping the top of the drainpipe just below the drain and flange with the wrench and turning until it loosens. However, if there are bolts or nuts on the underside of the sink, you can instead loosen these. Often times nuts or bolts are used to hold a disposal up to the sink drain. Gently push these bolts upward.
Return to the top of the sink and gently lift the flange out of the sink. The flange is identified by how it frames the sink drain. Sink flanges are often made out of metal. in some cases the drain cover may be attached to the flange. You may need to use a flathead screwdriver to gently lift up the flange. Do not be surprised if the drain grates comes up as well.
Inspect the now visible sink drain gasket for cracks or tears. If the gasket is worn, torn, or dried out replace with a fresh sink drain gasket. Position the new gasket into place so it surrounds the drain opening.
Remove the old dried up plumbers putty from the bottom of the flange and also around the inside of the sink opening where the gasket meets the opening. Apply a fresh amount of plumbers putty to these sections. Replace the flange so it fits flush around the drain.
Go underneath the sink and apply a small amount of plumbers putty where the underside of the flange or drain meets the garbage disposal if you have one. Push the disposal up gently and retighten the nuts and bolts.
Double check that the drain, disposal, and flange are securely in place. Turn on the disposal to ensure the flange or drain components don’t come loose. Run the water for a few minutes while inspecting under the sink for drips or slow streams of water. If properly repaired the sink drain leaking will stop.