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Cognitive Radio (CR) is a promising technology that can alleviate the spectrum shortage problem by enabling unlicensed users equipped with CRs to coexist with incumbent users in licensed spectrum bands without inducing interference to incumbent communications. Spectrum sensing is one of the essential mechanisms of CRs that has attracted great attention from researhers recently. Although the operational aspects of spectrum sensing are being investigated actively, its security aspects have garnered little attention. In this paper, we describe an attack that poses a great threat to spectrum sensing. In this attack, which is called the primary user emulation (PUE) attack, an adversary's CR transmits signals whose characteristics emulate those of incumbent signals. The highly flexible, software-based air interface of CRs makes such an attack possible. Our investigation shows that a PUE attack can severely interfere …