P2P mesh-pull live video streaming applications - -such as Cool-Streaming, PPLive, and PPStream - - have become popular in the recent years. In this paper, we examine the stream pollution attack, for which the attacker mixes polluted chunks into the P2P distribution, degrading the quality of the rendered media at the receivers. Polluted chunks received by an unsuspecting peer not only effect that single peer, but since the peer also forwards chunks to other peers, and those peers in turn forward chunks to more peers, the polluted content can potentially spread through much of the P2P network. The contribution of this paper is twofold. First, by way of experimenting and measuring a popular P2P live video streaming system, we show that the pollution attack can be devastating. Second, we evaluate the applicability of four possible defenses to the pollution attack: blacklisting, traffic encryption, hash verification, and chunk signing. Among these, we conclude that the chunk signing solutions are most suitable.