This paper investigates the physical-layer security (PLS) of a single-input-multiple-output (SIMO) underlay cognitive radio network (CRN) over cascaded - fading channels. A secondary user (SU) transmitter communicates with a secondary user receiver over cascaded - channels. Moreover, an eavesdropper residing in the SUs transmission’s range would be capable of intercepting the SUs’ transmission. The SU destination and the eavesdropper are both assumed to be equipped with multiple antennas. Both of the receivers adopt maximal-ratio combining (MRC) over the multiple copies of the signal. In an underlay CRN, the SU transmitter should keep adjusting the transmission power to ensure not to disturb the primary users’ (PUs) transmission. PLS is analyzed in terms of the secrecy outage probability ( SOP ) and the probability of non-zero secrecy capacity . Results indicate the evident effect of the cascade level and the number of antennas at the eavesdropper over the secrecy of the SUs pair. In addition, results reveal that PLS can be strengthened by increasing the number of antennas at the legitimate receiver. The impact of the interference level tolerable by the PU receiver over SUs’ secrecy is also analyzed. Monte-Carlo simulations and analytical results are presented to assess the system performance.