Cognitive Radio Networks (CRNs) emerge as a possible solution for the lack of spectrum, and make different networks merge. However, security becomes the key problem need to be solved. Cognitive radio networks face not only traditional network security intrusion problems, but also unique security risks. In this paper, we investigate security issues in different classes which including spectrum security and security of network merge. Countermeasures are given from three aspects: security mechanism, key management and identity authentication.