Cognitive Radio (CR) can improve the utilization of the spectrum by making use of licensed spectrum. However, the security aspects of cognitive radio networks have garnered little attention. In this paper, the authors discuss a threat to cognitive radio networks, which they call the Spectrum Sensing Data Falsification (SSDF) attack. Specifically, malicious users can always send presence and absence of information for the authorized user, or always report the opposite sensing results. Collaborative sensing performance was deduced under different attack scenarios.