A 51 percent attack happens when one group controls more than half of a blockchain's mining power, or hash rate. With that majority, the attackers can rewrite recent transaction history. The most common motive is double-spending: sending coins to an exchange, waiting for confirmation, withdrawing another asset, then broadcasting a secret longer chain that erases the original deposit. The exchange is left holding worthless tokens while the attacker walks away with both the withdrawn asset and their original coins.

The attack does not break cryptography. Private keys stay safe, and nobody can spend coins they do not own. The damage is to transaction history and trust. Small proof-of-work chains are the usual targets because hash rate can be rented cheaply on marketplaces like NiceHash. Ethereum Classic suffered several reorganizations in 2019 and 2020. Bitcoin SV was hit in 2021. Bitcoin itself remains impractical to attack because the cost of renting enough hash power is enormous.

What a 51% attacker can and cannot do

• Reverse their own recent transactions — yes, through chain reorganization
• Censor specific transactions — yes, by refusing to include them
• Steal coins from other wallets — no, private keys remain secure
• Create new coins out of thin air — no, protocol rules reject invalid blocks

Proof-of-stake networks face a similar risk if a validator controls a majority of staked tokens, but slashing penalties make the attack financially self-destructive.