Spectrum sensing in cognitive radio networks (CRNs) is subjected to some security threats such as primary user emulation (PUE) attack. In PUE attack, malicious users (MUPUE) transmit an emulated primary signal throughout the spectrum sensing interval of secondary users (SUs) to prevent them from accessing the primary user (PU) spectrum band. The performance of spectrum sensing under PUE attack is studied for two types of energy detectors: conventional energy detector (CED) and improved energy detector (IED). In spectrum access, a hybrid spectrum access scheme (a combination of overlay mode and underlay mode) is often promising. A novel analytical expression for SU network throughput under the PUE attack is developed under hybrid spectrum access in this chapter. A threshold optimization technique is studied to reduce the sensing error and improve the network throughput in the presence of attacker. Impact of several parameters such as sensing time, IED parameter, probability of attacker’s presence, attacker strength, maximum allowable SU transmit power, and tolerable interference limit at PU on the SU throughput is investigated. A simulation model is developed based on MATLAB to support our analytical formulations. © Springer International Publishing AG, part of Springer Nature 2019.