Remove both the front and rear nut (there is also one at the back).

Clean them both nuts and studs well to remove any oil.....you can start by using a degreaser like Purple Power or Krud Kutter and rinse it off with water.....then clean them with something better like Brake Clean or a similar solvent.

Dry everything off - then apply a small dab of RTV or a similar sealer.  Apply a small amount to the stud threads and a small dab on the base of the nut.

Tighten the nut down.....end of leak.

(Anti seize paste may also work as a sealant).