51% Attack

A 51% attack becomes possible the moment a single miner, mining pool, or staking entity controls more than half of a blockchain's total validating power. That threshold matters because most blockchains assume honest participants always hold the majority; once an attacker crosses it, they can outpace every other node and dictate which version of the transaction history the network accepts.

In practice, a majority attacker cannot forge signatures or steal coins from wallets they do not control. What they can do is narrower but still damaging: block chosen transactions from confirming, or rewrite recent blocks to erase a transaction after collecting the goods or exchange credit it paid for, the classic double spend. The longer the attacker sustains majority control, the deeper the reorganization they can force.

Smaller proof-of-work networks are the most frequent targets, since renting enough mining power is cheap relative to their market value. Ethereum Classic has suffered several confirmed attacks, including a 2020 incident that cost an exchange millions of dollars in reversed deposits, and Bitcoin Gold lost a comparable amount in 2018. Bitcoin itself has never been successfully attacked; its hash rate is now so large that acquiring or renting the needed hardware would cost far more than any realistic payout.

Proof-of-stake chains face a different calculus: an attacker must buy and lock up over half the staked supply, and slashing penalties destroy that stake once the attack is detected, making the exercise largely self-defeating. Common defenses include waiting for extra confirmations before treating a payment as final, and finality rules that make deep chain rewrites progressively more expensive.