To enhance the efficiency and the security in an anonymous channel of wireless systems, Yang et al. proposed a secure and efficient authentication protocol for an anonymous channel in wireless communications. The protocol did not use asymmetric cryptosystems to protect the identity of the mobile user because the cryptosystems have high complex and high computation cost. Not only can the protocol resist some security attacks but also the computation cost is low. However, the authors shall point out that their protocol is vulnerable to key compromise attack. Through this attack, any adversary can obtain the secret key Kh, i shared between the home network (HN) and the mobile station (MS). Then, the adversary can wiretap any messages that he/she wants using the secret key between the HN and the MS. Copyright © 2012 John Wiley & Sons, Ltd.