  In 2013, Tsai et al. cryptanalyzed Yeh et al. scheme and shown that Yeh et
al., scheme is vulnerable to various cryptographic attacks and proposed an
improved scheme. In this poster we will show that Tsai et al., scheme is also
vulnerable to undetectable online password guessing attack, on success of the
attack, the adversary can perform all major cryptographic attacks. As apart of
our contribution, we have proposed an improved scheme which overcomes the
defects in Tsai et al. and Yeh et al. schemes.
