51% Attack

A 51% attack is an assault on a blockchain in which one party gains majority control of the network's block-producing power — over half the hash rate on a proof-of-work chain, or of staked value on proof of stake — and uses it to rewrite recent history. With a sustained majority, an attacker can secretly mine a private branch, spend coins on the public chain (for example, depositing to an exchange and withdrawing other assets), then release the longer private branch, erasing the original deposit: a double-spend.

The attack has sharp limits that beginners often overestimate. A majority attacker cannot steal other people's coins, forge signatures, change the supply, or alter the rules — full nodes reject invalid blocks no matter who mines them. The power is confined to reordering or excluding recent transactions and double-spending the attacker's own funds. For large networks the cost is prohibitive: acquiring a majority of Bitcoin's hash rate would require billions in hardware and energy. Small chains are another story — Ethereum Classic was successfully 51%-attacked multiple times in 2019 and 2020 using rented hash power, with exchanges losing millions to double-spent deposits.

On proof-of-stake Ethereum, an analogous attacker would need a huge fraction of all staked ETH, and the protocol can slash identifiable attackers, destroying their stake — making the attempt not just expensive but self-immolating. The practical defense for users and exchanges is confirmation depth: requiring more confirmations on chains whose security budget is small relative to the values being transferred.