Cryptanalysis of the Yang -Wang's password authentication schemes

In 1999, Yang and shieh proposed two password authentication schemes using smart cards. But in 2003, Sun and Yeh indicated that their schemes are subject to the forgery attack. So in 2005, Yang and Wang proposed an improvement of Yang and Shieh’s schemes to resist against Sun and Yeh’s attack. However in this paper, we will point out that Yang and Wang’s schemes still suffer from the forgery attack. Because in their schemes, one can masquerade as a legal user and cheat the remote server successfully in the authentication phase.