In this paper, we investigate the physical layer security in cooperative non-orthogonal multiple access (NOMA) networks over Nakagamim fading channels. Both amplify-and-forward (AF) and decode-and-forward (DF) protocols are studied. More particularly, closed-form exact and asymptotic expressions for strictly positive secrecy capacity are provided considering NOMA-AF and NOMA-DF relaying systems. Numerical results are presented to justify the accuracy of the obtained theoretical analysis. These results show that NOMA-AF and NOMA-DF relaying networks have a similar secrecy performance.