You might get lucky and solve the problem just by tightening the fittings to the faucet with a basin wrench, which is usually the only tool that can reach up there. If not, replace the connections with braided stainless flex tubing. Once the leak is history, treat the mold with a strong bleach solution (using gloves, eye protection, and ventilation, of course). Then dry out the cabinet. If the wood is rotted, you may have to patch the bottom with plywood or replace the whole cabinet.