Unified Modeling Language (UML) is an effective tool to model information systems and to develop effective system specifications. Context Aware systems possess a dynamic behaviour based on parameters such as time and location which makes the task of modelling a context aware system quite complex namely the security requirements of the system. Since UML doesn't provide specific use cases to model Context Aware systems; in this work the authors propose suitable extensions to UML use case modelling to model the security requirements of a Context Aware system. This work is motivated and defined in terms of notations and diagramming techniques and the proposed extensions account for both static and dynamic functions of Context Aware systems.