Firstly, we mention an initial qualitative risk assessment, carried out by interviewing the operating manager of a large suspension bridge and a contractor responsible for part of a large subway tunnel network who want to use wireless sensor networks. The core of the talk deals with assessing the practical security of the particular COTS system adopted by our team, the Crossbow MICAz motes running TinyOS or XMesh, together with the Stargate gateway: we designed and implemented a variety of attacks on this system and we discuss the security problems we found, together with appropriate fixes where possible. While some of our attacks exploit generally known vulnerabilities, others like selective jamming and power exhaustion through routing table manipulation are original and interesting in their own right. In section we also demonstrate how an attacker can undetectably alter messages in an IEEE 802.15.4 radio environment.