  In 2004, W. C. Ku and S. M. Chen proposed an efficient remote user
authentication scheme using smart cards to solve the security problems of Chien
et al.'s scheme. Recently, Hsu and Yoon et al. pointed out the security
weaknesses of the Ku and Chen's scheme Furthermore, Yoon et al. also proposed a
new efficient remote user authentication scheme using smart cards. Yoon et al.
also modified the password change phase of Ku and Chen's scheme. This paper
analyzes that password change phase of Yoon et al's modified scheme is still
insecure.
