51% Attack
Controlling a majority of a network's mining or staking power to rewrite recent history — cheap against small chains, absurd against Bitcoin.
A 51% attack means controlling most of a chain’s hash rate (or stake), letting the attacker mine a private chain that outpaces the public one, then release it — reorganizing recent blocks to reverse their own transactions (double-spending). It cannot steal others’ coins or fake signatures; it attacks transaction ordering, not ownership.
Economics decide feasibility: against Bitcoin the hardware and energy required are nation-scale and the payoff self-defeating; against small PoW chains, rentable hash power has enabled repeated real attacks (Ethereum Classic, 2019–20). It’s why exchanges demand more confirmations from smaller chains — and why security budget, not TPS, is a chain’s real spec sheet.