A 51% attack (also called a majority attack) occurs when a single entity or coordinated group gains control of more than 50% of a blockchain network's mining power or staking authority. This majority control allows the attacker to manipulate the network.
What an Attacker Can Do
Double Spending: Spend the same cryptocurrency twice by reversing confirmed transactions.
Block Reorganization: Replace recently confirmed blocks with their own version of the chain.
Transaction Censorship: Prevent specific transactions from being confirmed.
What an Attacker Cannot Do
A 51% attack cannot create new coins out of thin air, steal coins from wallets, or change the fundamental rules of the protocol. It can only manipulate the order and inclusion of recent transactions.
Real-World Examples
Smaller proof-of-work blockchains like Ethereum Classic, Bitcoin Gold, and Verge have suffered 51% attacks. Bitcoin and Ethereum are considered effectively immune due to the enormous cost of acquiring majority control of their networks.