Publish/Subscribe (pub/sub) systems operating in hostile multi-domain environments face various types of internal and external threats. This paper focuses on internal threats manifested when pub/sub brokers misbehave by deviating from their prescribed protocol. Broker misbehavior may partially or fully compromise the pub/sub service integrity. In this paper, we exploit the notion of overlay neighborhoods and develop a solution to protect content-based pub/sub systems against misbehaving (a.k.a., byzantine) brokers. Our approach gives correct brokers the ability to oversee the actions of their neighbors, identify cases of deviation from the protocol and take steps to subvert the threat. As a result of this collective oversight, the system can preserve its service integrity despite presence of a configurable number of byzantine brokers. We have implemented our solution and report on our experimental evaluation results.