In open environments, such as mobile peer-to-peer systems, participants may need to access resources from unknown users. A critical security concern in such systems is the access of faulty resources, thereby wasting the requester’s time and energy and possibly causing damage to her system. A common approach to mitigating the problem involves reputation mechanisms; however, since reputation relies on cooperation, a reputation mechanism’s effectiveness can be significantly diminished in hostile environments. Reputation systems also require substantial communication among peers leading to: i) vulnerability to errors caused by intermittent connectivity; ii) message delivery disruptions caused by malicious peers; and iii) energy sapping message overheads. In this paper, we present AREX, a low-cost, adaptive mechanism designed to provide security for peers in hostile and uncertain environments, which are common in mobile P2P systems. AREX features an adaptive exploration strategy that ...