Sybil Attack

A Sybil Attack is a security threat in peer-to-peer networks, including blockchains, where an attacker creates multiple fake identities or nodes to gain outsized influence. Named after the book Sybil about a woman with multiple personalities, it exploits systems that assume one unique identity per participant.

The attack works by flooding the network with pseudonymous entities that the attacker controls. In blockchain contexts, the attacker might spin up thousands of low-cost nodes or accounts. These fake identities then sway consensus mechanisms, such as voting on transaction validity or governance proposals. For example, in a proof-of-stake network without strong identity checks, an attacker could amass stake under fake identities to control over 51% of voting power.

Sybil attacks matter because they erode decentralization and trust. They enable censorship, double-spending, or network forks by overpowering honest participants. Blockchains counter this with resource-based proofs: proof-of-work demands computational power per identity, while proof-of-stake ties influence to economic stake. Without defenses, networks risk centralization under attacker control.

Key characteristics include low cost for attackers in permissionless systems and types like direct attacks (altering consensus votes) or indirect attacks (disrupting data propagation). Real-world defenses often combine cryptographic identities, reputation systems, or economic penalties.

• Vulnerable systems: Pure one-node-one-vote protocols.
• Resistant examples: Bitcoin's proof-of-work.