51 Percent Attack
A 51 percent attack is when one miner or group controls most of a blockchain network's mining power and can disrupt transaction history.
Definition
A 51 percent attack happens when one miner, mining pool, or coordinated group controls more than half of a proof-of-work blockchain’s total mining power. With that majority, the attacker can influence which blocks are accepted by the network and can rewrite recent parts of the chain. This does not let them create coins from nothing, but it can let them reverse their own payments and damage trust in the network.
How It Works
Proof-of-work blockchains rely on miners competing to find valid blocks. The version of the blockchain with the most accumulated work is normally treated as the valid history. If an attacker controls most of the network’s hash rate, they can privately mine an alternative version of the chain faster than honest miners can extend the public one.
One common goal is a double spend. The attacker sends coins to a merchant or exchange, waits for the payment to be accepted, and privately mines a competing chain that leaves out that payment. If the attacker’s private chain becomes longer or has more accumulated work, they can publish it, causing the network to follow that version instead. The original payment disappears from the accepted history, while the attacker may still keep the goods, services, or exchanged assets.
A majority attacker can also censor transactions by refusing to include them in blocks they mine. They may slow down confirmations, reorganize recent blocks, or create uncertainty around final settlement. However, they cannot steal coins from wallets without private keys, change the protocol rules by themselves, or spend coins they do not control.
Why It Matters
51 percent attacks matter because blockchains depend on economic and computational difficulty to protect their history. When mining power is spread across many independent participants, rewriting the chain is expensive and risky. When hash rate is concentrated, rented cheaply, or pointed at a smaller network, the cost of attack can fall.
Large proof-of-work networks are generally harder to attack because they require enormous hardware, electricity, and coordination. Smaller networks using the same mining algorithm as a larger coin can be more vulnerable, especially if attackers can rent hash power temporarily. For miners, pools, exchanges, and users, the risk affects confirmation policies, pool decentralization, and confidence in finality.