51% Attack.

An attack where a single entity controls a majority of a blockchain's mining hash rate or staking power and uses it to rewrite history, double-spend, or censor transactions. It belongs to the Web3 & Blockchain category of cybersecurity.

An attack where a single entity controls a majority of a blockchain's mining hash rate or staking power and uses it to rewrite history, double-spend, or censor transactions.

In Proof-of-Work networks, an attacker with more than half of the global hash rate can outpace the honest chain, build a longer private fork, and reorganise blocks to double-spend coins. In Proof-of-Stake systems, the equivalent threshold is typically holding or delegating a majority of the staked supply. Beyond double-spending, attackers can censor specific transactions or temporarily halt confirmations, but they cannot steal arbitrary funds or break cryptographic signatures. Smaller chains and merge-mined coins are far more exposed because hash power can be rented. Defences include increased confirmation depth on exchanges, checkpointing, deeper finality (e.g. Casper FFG), economic slashing, and switching consensus algorithms.

Defences for 51% Attack typically combine technical controls and operational practices, as detailed in the full definition above.

Common alternative names include: Majority attack, Chain reorganisation attack.