An Access Control Model for Facebook-Style Social Network Systems

Recent years have seen unprecedented growth in the popularity of social network systems, with Facebook being an archetypical example. The access control paradigm behind the privacy preservation mechanism of Facebook is distinctly different from such existing access control paradigms as Discretionary Access Control, Role-Based Access Control, Capability Systems, and Trust Management Systems. This work takes a first step in deepening the understanding of this access control paradigm, by proposing an access control model that formalizes and generalizes the access control mechanism of Facebook. The model can be instantiated into a family of Facebook-style social network systems, each with a recognizably different access control mechanism, so that Facebook is but one instantiation of the model. We also demonstrate that the model can be instantiated to express policies that are not currently supported by Facebook, and yet these policies possess rich and natural social significance. Among these policies, we formally identify and characterize a special family of policies known as relational policies, which base their authorization decisions on the dynamic relationship between the resource owner and accessor. We believe the family of relational policies is a unique feature of social network systems. An executable encoding of this model has been developed to support experimentation with various instantiation of our access control model. This work thus delineates the design space of access control mechanisms for Facebook-style social network systems, and lays out a formal framework for policy analysis in these systems.