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Ubiquitous networks provide roaming service for mobile nodes enabling them to use the services extended by their home networks in a foreign network. A mutual authentication scheme between the roamed mobile node and the foreign network is needed to be performed through the home network. Various authentication schemes have been developed for such networks, but most of them failed to achieve security in parallel to computational efficiency. Recently, Shin et al. and Wen et al. separately proposed two efficient authentication schemes for roaming service in ubiquitous networks. Both argued their schemes to satisfy all the security requirements for such systems. However, in this paper, we show that Shin et al.'s scheme is susceptible to: (i) user traceability; (ii) user impersonation; (iii) service provider impersonation attacks; and (iv) session key disclosure. Furthermore, we show that Wen et al.'s scheme is also …