That depends on the state where you live, and if there is a "joint-and-several liability" law there. If there is such a law, an injured party could sue anyone associated with the bite, including you. Since you're now aware of the dog's presence, you could be held liable if the dog injured someone. Check with your insurance carrier to make sure that they'll still insure you if there's a pit bull living there. Some carriers have a clause that negates the policy if you have a Pit Bull, Rottie, or Chow-Chow. However, please don't assume that the dog is vicious simply because it's a pit bull. I work with pit bulls every single day at a Humane Society shelter, and I can honestly tell you, the breed has gotten a bad rap. Out of the hundreds of pits that we've had in our shelter, I haven't seen a single one that was outright vicious. We've had tons of bites and injuries caused by Chihuahuas, Cocker Spaniels, Daschunds, and Dalmatians, but no one has been bitten by a Pit Bull. It's up to you what you do with the tenant, but I just wanted to make sure you knew that beforehand.