In the rest of this article we first briefly overview the physical layer security issues in cognitive radio networks. We discuss
several known security attacks existing in cognitive radio networks and survey the related countermeasures on how to
defend against these attacks. Then we investigate one of the most important physical layer security parameters, the secrecy
capacity of a cognitive radio network, by studying the probability density function of the secrecy capacity and obtaining
the outage probability of secrecy capacity. At the end, we provide performance results and summarize the study of physical
layer security in cognitive radio networks.