# 51% Attack

> An attack where one party controls more than half of a network's mining power or stake and can rewrite recent history.

Canonical URL: https://fudfomo.co/glossary/51-attack
Source: What The Block! Dictionary v1.0 (last updated 2026-04-25), browsable at https://wtb.fudfomo.co.

## Definition

A 51 percent attack is what happens when a single party gains majority control of a proof-of-work network's hash rate, or a proof-of-stake network's staked coins. With that majority, they can refuse to confirm certain transactions and rewrite recent blocks, which can be used to double-spend.

Large networks like Bitcoin and Ethereum are very expensive to attack this way. Smaller proof-of-work coins have been hit several times, sometimes resulting in funds stolen from exchanges.

## Related terms

- [Mining](https://fudfomo.co/glossary/mining): Using computers to add new transactions to a blockchain and earn newly created coins as a reward.
- [Proof of Work](https://fudfomo.co/glossary/proof-of-work): A way to secure a blockchain by making computers do a lot of calculation. Used by Bitcoin.
- [Proof of Stake](https://fudfomo.co/glossary/proof-of-stake): A way to secure a blockchain using locked-up coins instead of computing power. Used by Ethereum.
- [Consensus](https://fudfomo.co/glossary/consensus): The way a blockchain's computers agree on which transactions are valid and which version of the ledger is correct.

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