  Any decentralised distributed network is particularly vulnerable to the Sybil
attack wherein a malicious node masquerades as several different nodes, called
Sybil nodes, simultaneously in an attempt to disrupt the proper functioning of
the network. Such attacks may cause damage on a fairly large scale especially
since they are difficult to detect and there has been no universally accepted
scheme to counter them as yet. In this paper, we discuss the different kinds of
Sybil attacks including those occurring in peer-to-peer reputation systems,
self-organising networks and even social network systems. In addition, various
methods that have been suggested over time to decrease or eliminate their risk
completely are also analysed along with their modus operandi.
