Trust based approaches have been widely used to counter insider attacks in wireless sensor networks because traditional cryptography-based security mechanisms such as authentication and authorization are not effective against such attacks. A trust model, which is the core component of a trust mechanism, provides a quantitative way to evaluate the trustworthiness of sensor nodes. The trust evaluation is normally conducted by watchdog nodes, which monitor and collect other sensors' behavior information. Most existing works mainly focus on the design of the trust models and how these models can be used to defend against certain insider attacks. However, these studies are empirical with the implicit assumption that the trust models are secure and reliable. In this paper, we discuss several security vulnerabilities that watchdog and trust mechanisms have, examine how inside attackers can exploit these security holes, and finally propose defending approaches that can mitigate the weaknesses of trust mechanism and watchdog.
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