A Sybil attack is a type of cyberattack in which one participant creates and controls many fake identities or accounts within a decentralised network. The goal is to make these identities appear as independent participants and use them to gain disproportionate influence over the network. The term is commonly associated with blockchain and peer-to-peer systems, where trust and decision-making are distributed among many participants.

In a cryptocurrency network, an attacker could create a large number of nodes or identities and attempt to influence communication, voting, transaction propagation, or other network activities. Simply creating multiple identities does not necessarily give an attacker control of a blockchain because well-designed consensus mechanisms use additional requirements to make influence costly or difficult to obtain.

For example, Proof of Work requires participants to contribute computational power, while Proof of Stake generally requires participants to commit cryptocurrency as stake. These mechanisms make it harder for an attacker to gain significant influence simply by generating thousands of fake identities. Other systems may use identity verification, reputation systems, resource requirements, or specialised consensus rules to reduce Sybil risks.

A successful Sybil attack can create several problems. An attacker may attempt to isolate users from honest nodes, manipulate information, disrupt peer-to-peer communication, or influence decentralised applications. However, a Sybil attack is different from a 51% attack: controlling many identities does not automatically mean controlling the blockchain's consensus.

Sybil resistance is therefore an important part of decentralised network security. By requiring participants to demonstrate scarce resources, stake, identity, or reputation, blockchain protocols can limit the influence that fake identities have and help preserve the network's security and decentralisation.