Cryptanalysis of a user authentication scheme using hash functions

Recently, Lee et al. proposed an improved scheme, called LLH scheme, to solve a security problem of guessing attack in the Peyravian-Zunic password scheme. The scheme comprises a password authentication protocol and a password change protocol. However, we discuss that the LLH scheme has several security problems: the scheme is still vulnerable to the guessing attack; the scheme cannot also prevent the stolen-verifier attack; the password change protocol in scheme is vulnerable to a denial of service attack.