51% Attack

A 51% attack happens when one party controls a majority of a blockchain's mining power or staked value and can therefore decide which blocks are accepted. With that control an attacker can censor transactions and rewrite recent history to spend the same coins twice, which is why it is also called a majority attack. It cannot create coins out of nothing, change balances in other people's wallets or break the network's cryptography, since ordinary nodes still reject invalid blocks. Large networks are protected mainly by cost: acquiring that much hash power or stake is expensive and would damage the value of the asset being attacked. Smaller chains with modest security budgets are the realistic targets, which is why exchanges require more confirmations on them.