Current peer-to-peer (P2P) streaming systems often assume that hosts are cooperative. However, this may not be true in the open environment of the Internet. In this paper, we discuss how to detect malicious hosts (e.g., with attacking actions and abnormal behavior) based on their history performance. In our system, each host monitors the performance of its neighbor(s) and reports this to a server. Based on the reports, the server computes host reputation with hosts of low reputation being malicious. A problem is that hosts may lie by submitting forged reports to the server. We hence formulate the reputation computing problem in the presence of lying hosts as a minimization problem and solve it by the traditional Levenberg-Marquardt algorithm. Simulation results show that our scheme can efficiently detect malicious hosts with high accuracy.