In order to solve security threat when external users directly access to internal sensor nodes in wireless sensor networks（ WSNs）

propose a simple session scheme based on two-factor user authentication scheme. In the scheme

it verifies validity of user by two factors

the smart card and the password

and determine state of user in session according to times interval that can be configured by administrator; then limit user’s actions based on the user’s status. This scheme effectively alleviate communication pressure in once-effective user authentication case

and avoid security risks in long-term effective user authentication case

it increases realizability for external users to inquire application-layer of inner node of WSNs.