51% attack

A 51% attack is when a single party controls more than half of a proof-of-work blockchain's mining power, or a large enough share of stake on a proof-of-stake chain, and uses it to rewrite recent history. The attacker can secretly mine a longer chain, then release it to reverse their own recent transactions. This lets them double-spend coins they already sent to an exchange.

An attacker cannot steal coins from other people's addresses or create coins beyond the protocol's rules, because nodes would reject invalid blocks. The main harm is reversing payments and undermining trust.

Smaller proof-of-work coins are most exposed because hash power can be rented. Ethereum Classic suffered several 51% attacks in 2019 and 2020. For Bitcoin, the hardware and energy needed make such an attack extremely costly.