A “51% attack” is shorthand for a majority attack on a proof-of-work blockchain: one actor or coordinated group has more than half of the network’s effective hash power for long enough to build a competing chain. That can make recent blocks vulnerable to reorganization and may enable double-spending the attacker’s own payments or censoring transactions. It does not let the attacker forge another user’s signature or spend coins from a wallet they do not control.
What majority hash power can do
In proof of work, nodes follow valid blocks with the greatest accumulated work. A majority miner can privately mine an alternative history that excludes a transaction it previously made, then publish that chain if it has accumulated more work. This can reverse a recent payment and allow the same funds to be spent on the attacker’s preferred branch. A majority can also repeatedly omit or delay selected transactions.
The term does not mean every attempted reorganization succeeds instantly. Outcomes depend on how much of the network’s hash power the attacker controls, how long the attempt lasts, the competing miners’ work, and how many confirmations the recipient waits for. Smaller, lower-hash-rate networks can face different economic conditions from Bitcoin. There is no timeless dollar cost that applies to all networks.
What it cannot do
Hash power does not replace private keys or a node’s consensus rules. A majority miner cannot create a valid signature for someone else, take coins from an unrelated address, or force fully validating nodes to accept a block that violates the protocol’s rules. The main leverage is over block ordering and recent history, not arbitrary control of every wallet or the token supply.
Proof of stake uses different thresholds
“51% attack” originated as a proof-of-work framing; it should not be applied mechanically to every proof-of-stake network. Ethereum’s documentation describes different stake thresholds for different outcomes: about one-third of stake can prevent finality under certain conditions; a special double-finality attack is described at 34% with severe slashing consequences; and a 66% supermajority can finalize a preferred chain. These are Ethereum-specific protocol scenarios, not universal rules for all proof-of-stake systems.
Mining-pool concentration also needs careful wording. A pool may coordinate block templates while individual miners provide hash power; the pool’s displayed share does not automatically mean its operator owns or permanently controls all of that hardware. Actual control and miners’ ability to redirect work matter.
Social Media Sentiment
A 2023 r/CryptoCurrency question asked what a majority-hash-power attacker could do beyond double-spending. Replies raised censorship and transaction-selection risks, while some commenters questioned whether a mining pool’s share means its operator controls the miners’ hardware. The thread is useful as an example of public questions and uncertainty, not a technical consensus or current risk assessment. Read the Reddit discussion.
Last updated: 2026-10
Related Terms
See Also
- How Do Crypto Bridges Work — And Why Are They Such a Hack Target?
- Are Layer 2 Networks Actually Decentralized?
- Why DAO Governance Votes Often Fall Short — and Why Protocols Still Use Them
- Swap crypto with ChangeNOW
Sources
- Bitcoin Developer Guide — Block Chain — Explains cumulative proof of work, chain reorganizations, and why majority hash power can make a 51 percent attack reliable.
- Bitcoin Developer Guide — Glossary — Defines a 51 percent attack in terms of majority hash rate, transaction history, and confirmation censorship.
- Ethereum.org — Proof-of-Stake Attack and Defense — Describes Ethereum-specific stake thresholds for finality delay, double finality, censorship, and chain control.