Fifty-One Percent Attack

Definition

A fifty-one percent attack is a commonly used name for an attack involving control of a majority of a proof-of-work network's effective mining power. An attacker with sustained majority hash rate can seek to outpace honest miners, reorganize recent history, censor transactions, or replace the attacker's own payments with conflicting spends. The label is shorthand rather than a precise boundary between possible and impossible attacks: outcomes also depend on duration, confirmation depth, coordination, and chance. Majority mining power does not allow an attacker to forge other users' signatures, spend arbitrary outputs, or create coins beyond consensus limits accepted by full nodes. The attack concerns selection among valid histories, not unlimited authority over validity rules. Mining concentration therefore matters, but counting mining devices or nodes is not equivalent to measuring control of the work used to extend the chain.