The continuous growth of wireless networks calls for more and more sophisticated solutions for their security. In particular, mechanisms for limiting effects of routing protocol attacks are becoming a mandatory requirement: black-hole and gray-hole attacks can in fact seriously compromise the performance of a critical infrastructure like a Wireless Mesh Network. In this paper we present a new routing metric aimed at mitigating the effects of such attacks, based on an estimation of the trustworthiness level of network nodes. By applying the metric to existing wireless routing protocols we show that it is possible to increase both the security level and the performance of the overall network, even in the presence of routing attacks.