A Sybil attack is a type of cyberattack that subverts the reputation system of a computer network service. The attacker creates large numbers of pseudonymous identities and uses them to gain disproportionate influence. The goal is to gain access to large amounts of information and control the network.
Sybil attacks are typically performed by tricking a local node into thinking it is communicating with many distinct remote nodes. By manipulating the results of these attacks, the attacker is able to control a large swath of the system. The attack has a negative impact on peer-to-peer networks that rely on group decisions. It can also be used to attack an honest entity.