What Is a 51% Attack?

July 31, 2025

A 51% attack is a type of blockchain security breach where a single entity or group gains control of more than 50% of the computing power (hashrate) or staking power of a blockchain network. This control gives the attacker enough influence to manipulate the blockchain in harmful ways.

🔧 How It Works

Most blockchains (like Bitcoin and Ethereum pre-merge) rely on proof-of-work (PoW) consensus, where miners solve complex mathematical problems to validate transactions and add new blocks. If a malicious actor controls the majority (51% or more) of the network’s power, they can:

Double-spend coins – spend the same cryptocurrency twice.

Censor transactions – prevent new transactions from gaining confirmations.

Reverse transactions – undo their own recent transactions, enabling fraud.

Stop miners – prevent other miners from mining valid blocks.

🔒 What They Can't Do

Even with 51% control, attackers cannot:

Steal coins from others' wallets.

Create coins out of thin air.

Change the rules of the protocol (without consensus).

🧪 Real-World Examples

Several smaller cryptocurrencies have suffered from 51% attacks:

Ethereum Classic (ETC) in 2020 had multiple 51% attacks.

Bitcoin Gold and Verge were also attacked in previous years.

Larger networks like Bitcoin are more secure due to the massive amount of computing power required to pull off such an attack—making it economically and logistically impractical.