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Part 5: Social NetworksInternational audienceOnline Social Networks (OSN) are increasingly becoming victims of Sybil attacks. These attacks involve creation of multiple colluding fake accounts (called Sybils) with the goal of compromising the trust underpinnings of the OSN, in turn, leading to security and the privacy violations. Existing mechanisms to detect Sybils are based either on analyzing user attributes and activities, which are often incomplete or inaccurate or raise privacy concerns, or on analyzing the topological structures of the OSN. Two major assumptions that the latter category of works make, namely, that the OSN can be partitioned into a Sybil and a non-Sybil region and that the so-called “attack edges” between Sybil nodes and non-Sybil nodes are only a handful, often do not hold in real life scenarios. Consequently, when attackers engineer Sybils to behave like real user accounts, these mechanisms perform poorly. In this work, we propose SybilRadar, a robust Sybil detection framework based on graph-based structural properties of an OSN that does not rely on the traditional non-realistic assumptions that similar structure-based frameworks make. We run SybilRadar on both synthetic as well as real-world OSN data. Our results demonstrate that SybilRadar has very high detection rate even when the network is not fast mixing and the so-called “attack edges” between Sybils and non-Sybils are in the tens of thousands

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Abstract. Multi-tenancy, elasticity and dynamicity pose several novel challenges for access control in mobile smartphone clouds such as the Android TM cloud. Accessing subjects may dynamically change, resources requiring protection may be created or modified, and a subject's access requirements to resources may change during the course of the application execution. Cloud tenants may need to acquire permissions from different administrative domains based on the services they require. Moreover, all the entities participating in a cloud may not be trusted to the same degree. Traditional access control models are not adequate for mobile clouds. In this work, we propose a new access control framework for mobile smartphone clouds. We formalize a trust-based access control model with delegation for providing fine-grained access control. Our model incorporates the notion of trust in the Role-Based Access Control (RBAC) model and also formalizes the concept of trustworthy delegation.

Effective access control is dependent not only on the existence of strong policies but also on ensuring that the access control enforcement subsystem is adequately protected. Protecting this subsystem has not been adequately addressed in the literature. In general, it is assumed to be implemented as a reference monitor in a trusted computing base (TCB) that is tamper-proof. However, in distributed access control, ensuring TCB security kernel to be tamper proof is not always feasible. It needs to be implemented in software and on platforms that can potentially have vulnerabilities. We posit that allowing a very limited opportunity to the attacker to enumerate exploitable vulnerabilities in the access control subsystem can considerably facilitate its protection. Towards this end we propose a moving target defense framework for access control in a distributed environment. In this framework, access control is provided by cooperation of several distributed modules that materialize randomly, announce their services, enforce access control and then disappear to be replaced by another module randomly. As a result, the attacker does not know which process can be targeted to compromise the access control system.