Dynamic spectrum access has proposed tiering radios into two groups: primary users and secondary users. PUs are assumed to have reserved spectrum available to them, while SUs (operating in overlay mode) must share whatever spectrum is available. The threat of emulation attacks, in which users pretend to be of a type they are not (either PU or SU) in order to gain unauthorized access to spectrum, has the potential to severely degrade the expected performance of the system. We analyze this problem within a Bayesian game framework, in which users are unsure of the legitimacy of the claimed type of other users. We show that depending on radios' beliefs about the fraction of PUs in the system, a policy maker can control the occurrence of emulation attacks by adjusting the gains and costs associated with performing or checking for emulation attacks.