In a recent paper (AMIS, 6(2S), 2012), Lee proposed a dynamic ID-based user authentication scheme based on smart card which is believed to have many abilities to resist a range of network attacks. I n this paper, we analyze the security of Lee's scheme and show that the scheme is in fact insecure against insider-assisted attack, even if the adversary doesn't know the secret information stored in the smart card. In addition, the adversary can perform off-line gue ssing attack and impersonation attack if the secret information stored in the smart card is compromised.