Majority Attack

A majority attack, usually called a 51% attack, is when one party controls more than half of a blockchain's mining power or staked value and uses it to rewrite recent history. The attacker cannot steal coins sitting in other people's wallets or change the protocol rules, because every node still checks signatures and validity. What they can do is censor transactions and reverse their own recent ones, which enables double spending against merchants and exchanges. Realistic targets are smaller networks with low hash rate or concentrated stake, since attacking a large chain means renting or buying an enormous share of its resources. The standard defence is waiting for more confirmations before crediting a deposit; exchanges often raise confirmation requirements or pause a market when a chain looks unstable.