  This paper reviews the Sybil attack in social networks, which has the
potential to compromise the whole distributed network. In the Sybil attack, the
malicious user claims multiple identities to compromise the network. Sybil
attacks can be used to change the overall ranking in voting applications,
bad-mouth an opinion, access resources or to break the trust mechanism behind a
P2P network. In this paper, different defense mechanisms used to mitigate Sybil
attacks are also reviewed.
