BitTorrent has become one of the most popular Internet applications, given the number of users and the fraction of the Internet traffic it consumes. Its wide adoption has exposed some potential problems, like selfish peer behavior. Related research efforts so far have focused on modeling the dynamics of swarms, as well as devising incentive mechanisms that improve fairness without sacrificing efficiency. To the best of our knowledge, this is the first paper to evaluate the impact of attacks that exploit BitTorrent vulnerabilities with the sole intention of harming a swarm. The paper sheds light on BitTorrent behavior by presenting state diagrams, describes two attacks, and then evaluates their negative impact in realistic swarm settings. To evaluate the impact of attacks, a discrete-event simulator was developed and validated against an experimental evaluation performed in a controlled environment. Our findings show the seriousness of the problem and should be the basis for the development of new mechanisms to increase BitTorrent security.