Dynamic and self organizing systems like those found in pervasive computing systems or semantic web based scenarios raise numerous challenges regarding security and privacy. Service discovery is a basic feature of SOA deployment in such systems, given that entities need to locate services they can describe without an a priory knowledge. After inherent threats to service discovery in ubiquitous networks, we propose a registry based solution in which context-aware security policies are enforced in order to ensure privacy and access control for clients and services. We offer the possibility for the users to specify their security preferences that will be enforced during the discovery process. Experimental results based on an implementation of our approach are finally presented.