A 51% attack is a blockchain security threat where a single actor or coordinated group controls more than half of a network’s consensus power, such as mining hash rate in proof of work or stake in proof of stake. With a majority share, the attacker can influence which transactions are confirmed and which version of the ledger becomes the accepted history.
How a 51% attack works
Most blockchains rely on the “longest” or most-work chain (or the chain with the most stake-backed attestations) as the canonical record. If an attacker consistently outpaces the rest of the network, they can build an alternative chain in parallel and later cause a chain reorganization, replacing recent blocks. This can enable double-spending, where the attacker pays a merchant, receives goods or services, then reorganizes the chain so the payment disappears while keeping the asset.
Majority control can also be used to censor transactions by refusing to include certain transfers in new blocks, delaying confirmations for specific users or contracts. However, even with 51% power, attackers generally cannot create coins from nothing, break cryptographic signatures, or spend funds from other wallets without access to private keys. The damage is mainly to transaction finality and trust in the ledger’s integrity.
Risks, examples, and defenses
51% attacks are more feasible on smaller networks with lower total hash rate or concentrated staking. In proof of work, renting hash power or redirecting mining capacity can temporarily give an attacker an edge. In proof of stake, large stake concentration or compromised validators can create similar risks.
Practical mitigations include increasing decentralization of miners or validators, using stronger finality mechanisms, monitoring for reorgs, and requiring more confirmations for high-value payments. This concept matters because it highlights how blockchain security depends not just on code, but on the distribution of real-world economic and computational power behind consensus.