Crypto glossary
51% attack
A 51% attack happens when one party controls most of a blockchain's mining power or stake and uses it to rewrite recent blocks or censor transactions.
Proof-of-work blockchains follow the chain with the most accumulated work. If a single miner or group controls more than half of the hashrate, it can build a private chain faster than everyone else and then release it, replacing recent blocks. That lets the attacker reverse its own payments, enabling double spending, or exclude other people's transactions.
Large networks like Bitcoin are extremely expensive to attack this way, but smaller proof-of-work coins have been hit repeatedly, including Ethereum Classic in 2019 and 2020 and Bitcoin Gold in 2018, sometimes using hashrate rented by the hour. Proof-of-stake chains face an equivalent threat from whoever controls a majority of staked tokens.
Even a majority cannot steal coins from addresses whose keys it lacks or create coins beyond the rules. The practical defences are waiting for more confirmations on small chains and watching for heavy concentration in a few mining pools or staking providers.
Example
If a coin's network runs at 10 TH/s and renting 12 TH/s for six hours costs less than a $500,000 exchange deposit it could reverse, the chain is a tempting 51% attack target.
Related terms
Latest articles on 51% attack
- Hackers Launched Second 51% Attack on Vertcoin Blockchain Targeting Crypto Exchange Bittrex
- [DASH] – Dash Core Launches v0.14 on Mainnet Bolstering Network Security Against 51% Attacks
- Ethereum Could be Vulnerable to 51% Attacks Due to Unpatched Mining Clients
- [Dash] – Dash Core Group Gives Statement on 51% Attack Vulnerability and Seeks To Calm Investors
