By sharing a number of sensing results to make a decision,the cooperative spectrum sensing alleviates the performancedegradation in cognitive radio systems. Due to its openness, onthe other hand, the cooperative spectrum sensing is vulnerableto security attacks which may cause serious degradation in theaccuracy of the spectrum sensing process. Among the varioustypes of attacks, spectrum sensing data falsification (SSDF)attacks [1] control some sensors and manipulate the sensingresults simultaneously for their own purpose, leading to awrong final decision at the fusion center.