In this paper, we consider User Authentication (UA) for wireless sensor networks. UA is a fundamental issue in designing dependable and secure systems. Imagine that a wireless sensor network is deployed in an intelligent building, a hospital, or even a university campus, to allow legitimate users to send queries and retrieve the respective result at any of the sensor nodes. Importantly, the system needs to provide a means of user authentication to verify if the user is valid. We propose a dynamic strong-password based solution to this access control problem and adapt it into a wireless sensor network environment. The proposed strong-password authentication approach imposes very light computational load and requires simple operations, such as one-way hash function and exclusive-OR operations. We present the design of the proposed scheme and discuss how to make use of the security features on MAC sublayer (Medium Access Control) based on the IEEE 802.15.4 specification. Analysis on security and communication costs is presented to evaluate the effectiveness of the proposed scheme.