Due to the prevalence of insecure open 802.11 access points, it is currently easy for a malicious party to launch a variety of attacks such as eavesdropping and data injection. In this paper, we consider a particular threat called the evil twin attack, which occurs when an adversary clones an open access point and exploits common automatic access point selection techniques to trick a wireless client into associating with the malicious access point. We propose two lines of defense against this attack. First, we present an evil twin detection strategy called context-leashing based upon recording the nearby access points when first associating with an access point. Using this contextual information, the client determines if an adversary has setup an evil twin access point at a different location. Next, we propose an SSH-style authentication method called EAP-SWAT to perform one-way access point authentication that fits into the extensible authentication protocol (EAP) framework.