Web-based social networks (WBSNs) are online communities where participants can establish relationships and share resources across the Web with other users In recent years, several WBSNs have been adopting Semantic Web technologies, such as FOAF, for representing users' data and relationships, making it possible to enforce information interchange across multiple WBSNs Despite its advantages in terms of information diffusion, this raised the need of giving content owners more control on the distribution of their resources, which may be accessed by a community far wider than they expected. In this paper, we present an access control model for WBSNs, where policies are expressed as constraints on the type, depth, and trust level of existing relationships Relevant features of our model are the use of certificates for granting relationships' authenticity, and the client-side enforcement of access control according to a rule-based approach, where a subject requesting to access an object must demonstrate that it has the rights of doing that.