Sensor nodes and actuators are becoming ubiquitous and research efforts focus on addressing the various issues stemming from resources constraints and other intrinsic characteristics typically associated with such devices and their applications. In the case of wearable nodes, and especially in the context of e-Health applications, the security issues are exacerbated by the direct interaction with the human body and the associated safety and privacy concerns. This work presents a policy-based, unified, cross-platform and flexible access control framework. It adopts a web services-compliant approach to enable secure and authorized fine-grained access control to body sensor network resources and services. The proposed scheme specifically considers the very limited resources of so-called nano nodes that are anticipated to be used in such an environment. A proof-of-concept implementation is developed and a preliminary performance evaluation is presented.