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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 increases the system's utility for benign peers and decreases the systempsilas utility for malicious peers. AREX reduces vulnerabilities and energy costs by operating without communication between peers. Through simulation, we demonstrate AREX's ability to reduce energy costs, protect benign peers, and diminish malicious peers' motivation to attack in a variety of hostile environments.