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Smart card based remote user authentication is receiving more and more attention than ever with the popularization of computer network. Recently, Yoon et al. proposed a remote user authentication scheme using smart cards based on Hwang et al.'s. Its security depends on the one-way hash function. They claim that the scheme provides mutual authentication between the user and the remote system and can avoid the security problems of Hwang et al.'s. Nevertheless, in this paper we give a cryptanalysis of Yoon et al.'s scheme and point out that there exist several security flaws in their scheme. The scheme is also vulnerable and insecure. Then we propose an improved one which can avoid such flaws in Yoon et al's scheme.