Recently, Juang proposed an efficient password authenticated key agreement scheme using smart cards for the multi-server architecture. Juang's scheme was intended to provide mutual authentication and session key agreement. Herein, we show that Juang's scheme is vulnerable to a privileged insider's attack and is not easily reparable. Furthermore, it does not provide forward secrecy and the user eviction mechanism.

In 2003, Wu and Chieu proposed a scheme that was claimed to be an enhanced version of Sun's password authentication scheme. Recently, Wu and Chieu themselves showed that their scheme is vulnerable to a forgery attack and then proposed an improved scheme. Herein, we demonstrate that Wu-Chieu's improved scheme is still vulnerable to several attacks.