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Cognitive Radio (CR) networks are vulnerable to selfish attacks, where secondary users increase their accessing probability to enhance their own utilities, resulting in serious performance degradation in CR networks. Therefore, we formulate the secure access for CR networks into a static game called Back off Window Control Game (BWCG) and a repeated game with punishment mechanism based on the CSMA protocol. We have proven the existence of a Nash Equilibrium in the BWCG game and design a punishment mechanism to motivate selfish users not to perform selfish attacks in the repeated game. Simulation results show that the proposed scheme can improve the network performance and the punishment mechanism can efficiently prevent selfish attacks in CR networks.