Wireless Sensor Networks (WSN) are quickly gaining popularity due to the fact that they are potentially low-cost solutions, which can be used in a variety of application areas. However, they are also highly susceptible to attacks, due to both the open and distributed nature of the network and the limited resources of the nodes. In this paper, we propose a modular, scalable, secure and trusted networking protocol stack, able to offer self-configuration and secure roaming of data and services over multiple administrative domains and across insecure infrastructures of heterogeneous WSNs. The focus is on trusted route selection, secure service discovery, and intrusion detection, while critical parts of the security functionality may be implemented in low-cost reconfigurable hardware modules, as a defense measurement against side channel attacks.