When a P2P system has millions of concurrently active peers, there is the risk that it could serve as a DDoS engine for attacks against a targeted host. In this paper we describe two approaches to creating a DDoS engine out of a P2P system: the first involves poisoning the distributed index in the peers; the second involves poisoning the routing tables in the peers. For both approaches, the targeted host does not have to be a participant in the P2P system, and could be a web server, a mail server, or a user's desktop. We then examine these two poisoning attacks in Overnet, a popular DHT-based P2P file-sharing system. By using limited poisoning attacks of short duration on Overnet's indexing and routing tables, we create DDoS attacks against a targeted host. We find that with modest effort, both DDoS attacks can direct significant traffic from diverse peers to the target.