The intercept probability performance of decode-and-forward (DF) underlay cognitive radio threshold-based network is investigated in this paper. Here, a secondary source is transferring data to secondary destination via secondary relay cooperation. There are interference limitations on secondary user of cognitive system from the primary licensed users. The DF relays are not assumed to correctly decode the message always and only those relays which can satisfy the predetermined threshold are successful in decoding the source message. The worst case scenario has been studied, where Maximal Ratio Combining (MRC) and Selection Combining (SC) diversity reception schemes have been applied only at eavesdropper. We have obtained asymptotic and diversity analysis for balanced and unbalanced system. The secrecy of the system is shown to be significantly affected by eavesdropper link quality, predetermined threshold and interference power limitations. Furthermore, the optimal relay selection scheme (OS) is presented and the improvement in secrecy performance is shown with the increase in relay number.