The Sybil attack in computer security is an attack wherein a reputation system is subverted by forging identities in peer-to-peer networks. It is named after the subject of the book Sybil, a case study of a woman diagnosed with dissociative identity disorder. The name was suggested in or before 2002 by Brian Zill at Microsoft Research. The term "pseudospoofing" had previously been coined by L. Detweiler on the Cypherpunks mailing list and used in the literature on peer-to-peer systems for the same class of attacks prior to 2002, but this term did not gain as much influence as "Sybil attack".
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In a Sybil attack, the attacker subverts the reputation system of a peer-to-peer network by creating a large number of pseudonymous identities, using them to gain a disproportionately large influence. A reputation system's vulnerability to a Sybil attack depends on how cheaply identities can be generated, the degree to which the reputation system accepts inputs from entities that do not have a chain of trust linking them to a trusted entity, and whether the reputation system treats all entities identically. Evidence shows large-scale Sybil attack can be carried out in a very cheap and efficient way in realistic systems like BitTorrent Mainline DHT. 
An entity on a peer-to-peer network is a piece of software which has access to local resources. An entity advertises itself on the peer-to-peer network by presenting an identity. More than one identity can correspond to a single entity. In other words, the mapping of identities to entities is many to one. Entities in peer-to-peer networks use multiple identities for purposes of redundancy, resource sharing, reliability and integrity. In peer-to-peer networks, the identity is used as an abstraction so that a remote entity can be aware of identities without necessarily knowing the correspondence of identities to local entities. By default, each distinct identity is usually assumed to correspond to a distinct local entity. In reality, many identities may correspond to the same local entity.
A faulty node or an adversary may present multiple identities to a peer-to-peer network in order to appear and function as multiple distinct nodes. After becoming part of the peer-to-peer network, the adversary may then overhear communications or act maliciously. By masquerading and presenting multiple identities, the adversary can control the network substantially.
A notable Sybil attack (in conjunction with a traffic confirmation attack) was launched against the Tor anonymity network for several months in 2014 by unknown perpetrators. Many in the network security community suspect the NSA/CIA to be responsible for the attack, and some speculate that the attack may have been connected to the investigation into the Silk Road website.
Validation techniques can be used to prevent Sybil attacks and dismiss masquerading hostile entities. A local entity may accept a remote identity based on a central authority which ensures a one-to-one correspondence between an identity and an entity and may even provide a reverse lookup. An identity may be validated either directly or indirectly. In direct validation the local entity queries the central authority to validate the remote identities. In indirect validation the local entity relies on already accepted identities which in turn vouch for the validity of the remote identity in question.
Identity-based validation techniques generally provide accountability at the expense of anonymity, which can be an undesirable tradeoff especially in online forums that wish to permit censorship-free information exchange and open discussion of sensitive topics. A validation authority can attempt to preserve users' anonymity by refusing to perform reverse lookups, but this approach makes the validation authority a prime target for attack. Alternatively, the authority can use some mechanism other than knowledge of a user's real identity - such as verification of an unidentified person's physical presence at a particular place and time - to enforce a one-to-one correspondence between online identities and real-world users.
Sybil prevention techniques based on the connectivity characteristics of social graphs can also limit the extent of damage that can be caused by a given Sybil attacker while preserving anonymity, though these techniques cannot prevent Sybil attacks entirely, and may be vulnerable to widespread small-scale Sybil attacks. Examples of such prevention techniques are SybilGuard and the Advogato Trust Metric and also the sparsity based metric to identify Sybil clusters in a distributed P2P based reputation system.
- Douceur, John R. (2002). "The Sybil Attack" (PDF). International workshop on Peer-To-Peer Systems. Retrieved 23 April 2016.
- Oram, Andrew. Peer-to-peer: harnessing the benefits of a disruptive technology.
- Wang, Liang; Kangasharju, Jussi. (2012). "Real-world sybil attacks in BitTorrent mainline DHT". IEEE GLOBECOM. Retrieved 30 September 2013.
- Wang, Liang; Kangasharju, Jussi. (2013). "Measuring Large-Scale Distributed Systems: Case of BitTorrent Mainline DHT" (PDF). IEEE Peer-to-Peer. Retrieved 30 September 2013.
- O'Whielacronx, Zooko. "Levien's attack-resistant trust metric". <p2p-hackers at lists.zooko.com>. gmane.org. Retrieved 10 February 2012.
- Kurve, Aditya C. (2011). "Sybil Detection via Distributed Sparse Cut Monitoring". International Communication Conference ICC. Retrieved 31 March 2011.
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- Measuring Large-Scale Distributed Systems: Case of BitTorrent Mainline DHT by Liang Wang and Jussi Kangasharju. IEEE P2P 2013.
- Real-world sybil attacks in BitTorrent mainline DHT by Liang Wang and Jussi Kangasharju. IEEE GLOBECOM 2012.
- Sybil Attacks Against Mobile Users: Friends and Foes to the Rescue by Daniele Quercia and Stephen Hailes. IEEE INFOCOM 2010.
- On the establishment of distinct identities in overlay networks by Rida A. Bazzi and Goran Konjevod
- An Offline Foundation for Online Accountable Pseudonyms by Bryan Ford and Jacob Strauss
- A Sybil-proof one-hop DHT by Chris Lesniewski-Laas
- The Sybil Attack in Sensor Networks: Analysis & Defenses
- A Survey of Solutions to the Sybil Attack
- SybilGuard: Defending Against Sybil Attacks via Social Networks
- On Network formation: Sybil attacks and Reputation systems
- Trust Transfer: Encouraging Self-Recommendations without Sybil Attack
- A Survey of DHT Security Techniques by Guido Urdaneta, Guillaume Pierre and Maarten van Steen. ACM Computing surveys, 2009.
- An experiment on the weakness of reputation algorithms used in professional social networks: the case of Naymz by Marco Lazzari. Proceedings of the IADIS International Conference e-Society 2010.