Recent peer-to-peer research has focused on providing efficient hash lookup systems that can be used to build more complex systems. These systems hav good properties when their algorithms are executed correctly but have not generally considered how to handle misbehaving nodes. This paper looks at what sorts of security problems are inherent in large peer-to-peer systems based on distributed hash lookup systems. We examin the types of problems that such systems might face, drawing examples from existing systems, and propose some design principles for detecting and preventing these problems.