First talk with your manager. If necessary, talk to your manager's manager (or whoever has a say in it). Explain the situation. If possible, refuse to work on t

First talk with your manager. If necessary, talk to your manager's manager (or whoever has a say in it). Explain the situation. If possible, refuse to work on the "horrible mess". That's risky, but it's better than letting your name be associated with that project forever.