The great enthusiasm which accompanies the wide adoption of handled devices and remote services is very often attenuated by the number and frequency of security problems encountered by users and services providers. The ubiquitous (pervasive) nature of such applications has brought new security vulnerabilities. Security in pervasive computing is thus, still a hot topic.Context-based security is an emerging approach for modeling adaptive security solutions based on the situation of use of the system. Our contribution aims at presenting a new model for specifying context-based security policies. This approach is based on contextual graphs and relies on a set of contextual information collected from the system and user's environments.