What is a 51% Attack?

A 51% attack, or majority attack, occurs when a miner or group of miners controls over half of a blockchain network's total computational power (hash rate). This control allows them to potentially disrupt the network by preventing new transactions from confirming, halting payments between some or all users, or reversing transactions that were completed while they were in control (double-spending).

How it Works

With majority control, attackers can:

• Exclude or modify the ordering of transactions: They can prevent specific transactions from being confirmed.
• Prevent other miners from finding blocks: They can mine all subsequent blocks, effectively monopolizing block creation.
• Double-spend coins: They can reverse their own transactions to spend the same cryptocurrency multiple times.

Vulnerability

Smaller cryptocurrencies with lower overall hash rates are more vulnerable to 51% attacks as it requires less computational power (and thus less cost) to gain majority control.

Mitigation

Networks like Bitcoin, with vast amounts of distributed hashing power, are extremely expensive and difficult to attack in this manner. Decentralization is a key defense.

Related Terms

• Hash Rate
• Double Spending
• Proof of Work (PoW)
• Consensus Mechanism