Those are sores from skin ittitation, most likely caused from fungal dermatitis which is usually treated with a rabbit safe antifungal medication, or mange from burrowing mites which is usually treated with ivermectin or selamectin(Revolution/Stronghold). If your rabbit also has a sore on the bottom of the hind foot, your rabbit may have sore hocks that need treatment as well. You'll need to find a knowledgeable rabbit vet to get your rabbit the proper diagnosis and treatment.