Abstract. Recently, Shen et al. proposed an improvement on YangShieh’s timestamp-based password authentication scheme using smart cards. Then they claimed that their scheme cannot withstand a forged login attack, but also eliminate a problem of Yang-Shieh’s. However, their scheme is still susceptible to forged login attack. In this paper, we show how the forged login attack can be worked out on Shen et al.’s scheme and present an enhancement to resolve such a problem.