Overview
A 51% attack is a theoretical (and sometimes practical) vulnerability of decentralized blockchains. If an attacker controls the majority of the network's consensus power, they can manipulate the ledger in several ways.
Capabilities of an Attacker
- Double Spending: They can send a transaction, then mine a private chain that excludes that transaction and release it to the network, effectively 'undoing' the payment.
- Censorship: They can prevent certain transactions from being confirmed.
- Reorganizing the Chain: They can 'roll back' recent blocks to change history.
Limitations
An attacker cannot steal funds from addresses they don't own, change the total supply of coins, or alter the fundamental rules of the protocol (like block size).
Cost
For large networks like Bitcoin or Ethereum, the cost of a 51% attack is prohibitively high, involving billions of dollars in hardware or capital.