Security is top priority for pervasive applications, for which the number of reported incidents is in constant increase. Context-based security is an emerging approach. It aims at coping with the security problems resulting from the high heterogeneity and dynamicity of such environments. In this paper, we propose a new model for policies specification based on contextual graphs. It relies on a set of contextual information collected from the system and user's environments. This approach aims at providing a convenient way for modeling security requirements in complex systems.