Despite this strong collusive power, we have shown that the asymmetric information facing the collusive bidders can be exploited to significantly weaken their collusive power, by eliminating the scope of collusive arrangements that could make all collusive bidders uniformly better off regardless of their types. We show that the second-best outcome is achievable if a cartel is not all-inclusive (which will be the case either if there is a noncollusive bidder or if there are multiple bidding cartels), or if the outcome involves a nontrivial probability of the object not being allocated to any bidder. More generally, we have shown that the optimal collusion-proof auction rule involves a positive probability of the object not being allocated to a collusive bidder.