Majority Attack
A proof-of-work attack in which coordinated hash power can outpace the honest chain and reorganize recent valid history.
- Also known as
- 51% attack
A majority attack occurs when one actor or coordinated group controls enough active proof-of-work capacity to build a competing valid chain faster than the rest of the network for the period of the attack. "51%" is a common shorthand, not a magical threshold in every network condition.
The attacker may reorganize recent blocks, double-spend transactions it controls, censor selected transactions, or disrupt confirmation timing. It cannot produce valid signatures for unrelated keys, spend arbitrary outputs, exceed the permitted subsidy, or force full nodes to accept invalid consensus rules.
Risk depends on compatible hash-power availability, rental markets, pool coordination, attack duration, liquidity, confirmation policy, and the value available to steal. More confirmations increase attack cost but do not change weak operational controls at an exchange.
Proof-of-stake systems can face majority-stake or finality attacks, but their mechanics and recovery differ. Calling every validator attack a "51% attack" can hide the relevant stake, slashing, checkpoint, and governance assumptions.
Related terms
Explore connected entries beyond the alphabetical index.
Proof of Work (PoW)
→A consensus mechanism that makes block production require verifiable computational work.
Hash rate
→The number of proof-of-work hash attempts performed per second by mining hardware or an entire network.
Double Spend
→A double spend is an attempt to spend the same coins twice by getting one conflicting transaction accepted over another.
Confirmations
→The count of blocks confirming a transaction, usually including the block that first contains it.
Nakamoto Consensus
→Probabilistic consensus in which nodes validate rules and prefer the valid chain with the most accumulated proof of work.