WSNs are trending towards dynamic environments that enable multiple parties to concurrently deploy and exploit multiple applications on shared nodes. The node owners want to share their nodes' capabilities in order to increase return on investment, provide value added services, and easily share sensor node services. These concepts drive an evolving view on the software support that is required to service all stakeholders. For example, trust establishment, access control and security policy enforcement must be addressed. The node middleware must be extended to enable such shared usage of nodes while ensuring security. This paper presents the SecLooCI WSN middleware, which enables secure multi-party interactions on top of resource constrained sensor nodes. A prototype implementation for AVR Ravens running the Contiki OS shows the feasibility of the model for this low power micro-controller class of devices. This demonstrates that resource constrained sensor nodes are able to support secure node sharing.