What is a Sybil attack, and why is it hard to pull off on a blockchain?

A Sybil attack is one entity creating many fake identities to gain outsized influence over a network; blockchains resist it by making each identity's influence cost real resources.

The name comes from the idea of a single actor wearing many masks. On a naive voting system, you could just spin up a million fake accounts and outvote everyone. A blockchain defeats this by tying influence to something scarce and expensive rather than to identity count: under Proof of Work, every fake node must still perform the full, costly mining computation to matter; under Proof of Stake, every fake node must lock up real capital as collateral. Creating a thousand identities buys you nothing unless you also bring a thousand times the hardware or the stake, which is exactly the barrier that keeps the network honest. Sybil resistance is the core job that the consensus mechanism performs.

Go deeper:

• Sybil attack — Wikipedia — how one actor forging many identities subverts a naive network, and the defences against it.