In this correspondence, we investigate the physical layer security for cooperative non-orthogonal multiple access (NOMA) systems, where both amplify-and-forward (AF) and decode-and-forward (DF) protocols are considered. More specifically, some analytical expressions are derived for secrecy outage probability (SOP) and strictly positive secrecy capacity (SPSC). Results show that AF and DF almost achieve the same secrecy performance. Moreover, asymptotic results demonstrate that the SOP tends to a constant at high signal-to-noise ratio (SNR). Finally, our results show that the secrecy performance of considered NOMA systems is independent of the channel conditions between the relay and the poor user.