P2P mesh-pull live video streaming applications – such as CoolStreaming, 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 therendered media at the receivers. Polluted chunks received by an unsuspecting peer not only effectthat single peer, but since the peer also forwards chunks to other peers, and those peers in turnforward chunks to more peers, the polluted content can potentially spread through much of the P2Pnetwork. The contribution of this paper is twofold. First, by way of experimenting and measuring apopular P2P live video streaming system, we demonstrate 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 chunksigning solutions are most suitable.