A Robust Password-Based Multi-Server Authentication Scheme
  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. 
Vorugunti Chandra Sekhar, Mrudula Sarvabhatla
cs.CR