The Sybil Attack in Sensor Networks: Analysis & Defenses. James Newsome Elaine Shi Dawn Song Adrian Perrig Carnegie Mellon Carnegie Mellon Carnegie Mellon Carnegie Mellon University University University University jnewsomeQece.cmu.edu rshi @cmu.edu dawnsongQcmu.edu adrianQcmu.edu ABSTRACT Security is important for many sensor network applications. A particularly harmful attack against sensor and ad hoc networks is known as the Sybil attack [6], where a node illegitimately claims multiple identities. This paper system- atically analyzes the threat posed by the Sybil attack to wireless sensor networks. We demonstrate that the attack can be exceedingly detrimental to many important functions of the sensor network such as routing, resource allocation, misbehavior detection, etc. We establish a classification of different types of the Sybil attack, which enables us to bet- ter understand the threats posed by each type, and better design countermeasures against each type. We then propose several novel techniques to defend against the Sybil attack, and analyze their effectiveness quantitatively. Categories and Subject Descriptors C.2.2 [Computer-Communication Networks]: Network Protocols General Terms Algorithms, Security Keywords Sybil Attack, Sensor Networks, Security *This research was supported in part by the Center for Com- puter and Communications Security at Carnegie Mellon un- der grant DAAD19-02-1-0389 from the Army Research Of- fice, and by gifts from Bosch, Cisco, Intel, and Matsushita Electric Works Ltd. The views and conclusions contained here are those of the authors and should not be interpreted as necessarily representing the official policies or endorse- ments, either express or implied, of ARO, Bosch, Carnegie Mellon University, Cisco, Intel, Matsushita Electric Works Ltd., or the U.S. Government or any of its agencies. Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. To copy otherwise, to republish, to post on servers or to redistribute to lists, requires prior specific permission andlor a fee. ZPSN’O4, April 26-27, 2004, Berkeley, California, USA. Copyright 2004 ACM 1-581 13-846-6/04/0004 ... $5.00. 1. INTRODUCTION Sensor networks are a promising new technology to enable economically viable solutions to a variety of applications, for example pollution sensing, structural integrity monitoring, and traffic monitoring. A large subset of sensor network ap- plications requires security, especially if the sensor network protects or monitors critical infrastructures. Security in sensor networks is complicated by the broad- cast nature of the wireless communication and the lack of tamper-resistant hardware (to keep per-node costs low). In addition, sensor nodes have limited storage and computa- tional resources, rendering public key cryptography imprac- tical. In this paper, we investigate the Sybil attack, a particu- larly harmful attack in sensor networks. In the Sybil attack, a malicious node behaves as if it were a larger number of nodes, for example by impersonating other nodes or simply by claiming false identities. In the worst case, an attacker may generate an arbitrary number of additional node iden- tities, using only one physical device. Related Work The Sybil attack was first described by Douceur in the context of peer-to-peer networks [6]. He pointed out that it could defeat the redundancy mecha- nisms of distributed storage systems. Karlof and Wagner noted that the Sybil attack also poses a threat to routing mechanisms in sensor networks [9]. Contributions This is the first paper that systematically analyzes the Sybil attack and its defenses in sensor networks. This paper makes the following contributions. We intro- duce a taxonomy of the different forms of the Sybil attack as it applies to wireless sensor networks. We analyze how an attacker can use the different types of the Sybil attack to perturb or compromise several sensor network protocols. We propose several new defenses against the Sybil attack, including radio resource testing, key validation for random key predistribution, position verification, and registration. Through quantitative analysis, we show that the radio re- source testing method is very effective given the assumption that a malicious node cannot send on multiple channels si- multaneously. We also present a quantitative evaluation for the random key predistribution approach showing that it is robust to compromised nodes. In particular, we show that in the multi-space pairwise scheme storing 200 keys at each node, the attacker would have to compromise 400 nodes be- fore having even a 5% chance of being able to fabricate new identities for the Sybil attack. 2592. SYBIL ATTACK TAXONOMY We define the Sybil attack as a malicious device illegiti- mately taking on multiple identities. We refer to a malicious device’s additional identities as Sybil nodes. To better un- derstand the implications of the Sybil attack and how to defend against it, we develop a taxonomy of its different forms. We propose three orthogonal dimensions: direct vs indirect communication, fabricated vs stolen identities, and simultaneity. 2.1 Dimension I: Direct vs. Indirect Communication Direct Communication One way to perform the Sybil attack is for the Sybil nodes to communicate directly with legitimate nodes. When a legitimate node sends a radio message to a Sybil node, one of the malicious devices listens to the message. Likewise, messages sent from Sybil nodes are actually sent from one of the malicious devices. Indirect Communication In this version of the attack, no legitimate nodes are able to communicate directly with the Sybil nodes. Instead, one or more of the malicious de- vices claims to be able to reach the Sybil nodes. Messages sent to a Sybil node are routed through one of these ma- licious nodes, which pretends to pass on the message to a Sybil node. 2.2 Dimension 11: Fabricated vs. Stolen Identities A Sybil node can get an identity in one of two ways. It can fabricate a new identity, or it can steal an identity from a legitimate node. Fabricated Identities In some