With the outbreak of applications for smartphones, attempts to collect personal data without their user's consent are multiplying and the protection of users privacy has become a major issue. In this paper, an approach based on semantic web languages (OWL and SWRL) and tools (DL reasoners and ontology APIs) is described. The proposed semantic firewall takes its decisions (authorize or forbid some action) on the basis of a set of privacy protection rules grounded on two ontologies respectively modeling identity of mobile phone's users and privacy policies. To validate this ontology-based approach, a proof of concept involving a real privacy threat scenario is implemented in Java and the porting of the semantic firewall to the Android platform is outlined.