In order to remedy the security weaknesses of a robust user authentication framework for wireless sensor networks,an enhanced user authentication framework is presented.The enhanced scheme requires proof of the possession of both a password and a smart card,and provides more security guarantees in two aspects:1) it addresses the untraceability property so that any third party accessing the communication channel cannot link two authentication sessions originated from the same user,and 2) the use of a smart card prevents offline attacks to guess passwords.The security and efficiency analyses indicate that our enhanced scheme provides the highest level of security at reasonable computational costs.Therefore,it is a practical authentication scheme with attractive security features for wireless sensor networks.