An attacker who controls a computer in an overlay network can effectively control the entire overlay network if the mechanism managing membership information can successfully be targeted. This article describes Fireflies, an overlay network protocol that fights such attacks by organizing members in a verifiable pseudorandom structure so that an intruder cannot incorrectly modify the membership views of correct members. Fireflies provides each member with a view of the entire membership, and supports networks with moderate total churn. We evaluate Fireflies using both simulations and PlanetLab to show that Fireflies is a practical approach for secure membership maintenance in such networks.