A 51% attack is a situation in which one participant or a coordinated group controls a majority of the resource used to determine blockchain consensus. In Proof-of-Work networks, this resource is typically computational power. In Proof-of-Stake systems, an analogous threat involves controlling a large share of the stake, although the exact attack model depends on the protocol.

Majority control does not automatically provide complete control over a blockchain. An attacker normally cannot arbitrarily create coins, forge other users' signatures, or modify data that has already achieved strong finality. The primary risk is the ability to influence the production and ordering of new blocks and compete with the honest chain.

Potential attacker capabilities

  • attempting double-spending under suitable conditions;
  • censoring or delaying selected transactions;
  • influencing transaction ordering;
  • creating a competing chain and causing reorganizations where the protocol permits them.

The best-known scenario involves double-spending. An attacker makes a payment on one chain and then uses its controlled consensus resource to build an alternative history in which the payment does not exist. If the alternative chain becomes preferred under the protocol rules, the recipient may lose the basis for treating the original payment as final.

Protection depends on network architecture. In PoW, the distribution of hashrate and the economic cost of obtaining significant computational power are important. In PoS, the economic cost of controlling stake, slashing, finality, and other protocol mechanisms play a major role.

The risk is particularly relevant to smaller networks where acquiring a large share of the consensus resource may be economically easier. Decentralization and broad resource distribution are therefore important elements of blockchain security.