In this study, the secrecy performance of cooperative half‐duplex cognitive relay networks (CRNs) comprised of multiple primary users (PUs) and multiple eavesdroppers under the effect of unreliable wireless backhauls is studied. The authors consider relay networks with single relay assisting multiple sources under power constraints at PUs and secondary transmitters, and based on the availability of channel‐state information at the sources, they propose two best source selection schemes, namely: (i) optimal source selection (OSS) and (ii) sub‐optimal source selection (SoSS). For both scenarios, they obtain exact closed‐form and asymptotic expressions for the system's outage probability and carry out numerical simulations to justify their analyses. From the results, source selection is proven to be capable of counteracting the negative impact of unreliable backhaul on CRNs. The secrecy performance limitations are also verified to be influenced by the backhaul reliability. Furthermore, OSS is more desirable compared to SoSS in low signal‐to‐noise ratio (SNR) regime, while the secrecy performance for both schemes converge to an asymptotic limit in high‐SNR ranges.