  Recently, Zhang et al. proposed a single-state semi-quantum key distribution
protocol (Int. J. Quantum Inf, 18, 4, 2020) to help a quantum participant to
share a secret key with a classical participant. However, this study shows that
an eavesdropper can use a double C-NOT attack to obtain parts of the final
shared key without being detected by the participants. To avoid this problem, a
modification is proposed here.
