  Recently, Yang et al. (Quantum Inf Process 18, 74, 2019) proposed a two-party
quantum key agreement protocol over a collective noisy channel. They claimed
that their quantum key agreement protocol can ensure both of the participants
have equal influence on the final shared key. However, this study shows that
the participant who announces the permutation operation can manipulate the
final shared key by himself/herself without being detected by the other. To
avoid this loophole, an improvement is proposed here.
