In the CR networks, the unlicensed users can access the idle spectrum when they sense that licensed users are not using this spectrum. Due to the ability of dynamically reusing the spare spectrum resources, there is a remarkable increase of the spectrum utilization. However cognitive radio has brought new problems and concerns, especially in safety. Primary user emulation attack (PUEA) and Spectrum Sensing Data Falsity (SSDF) attack are typical attacks in spectrum sensing. In this paper we will give a detailed introduce of this two threats. Then we will represent some methods to weaken the effectiveness of these attacks.