What is a 51% attack?

A 51% attack is when malicious attackers attempt to take over a cryptocurrency’s network by gaining control of more than half—aka 51%—of a network’s mining hashrate. If attackers gain control of a network, they can falsify new transactions, spend coins they don’t have, prevent transactions from being validated, and more.

Some cryptocurrencies, including Bitcoin, rely on mining to secure the network and validate new transactions. Mining is the process of verifying and adding transactions to a blockchain network. In exchange for verifying and adding transactions, miners are rewarded with crypto.

Generally, the more miners participating in a network, the more difficult it is to successfully attack the network because there’s more computer power being used to process transactions. The amount of computing power being used by a network is referred to as its hashrate. You can learn more about hashrates in this article.

The fewer the miners, the lower the hashrate, and the more vulnerable the network.

One way to think about this is to picture miners as voters who can approve or invalidate a transaction. If you have 100 voters in a room, 51 or more of them would have to agree for a transaction to be validated. Even if 40 of the 100 voters were malicious attackers, they would still get outvoted by the 60 other voters.

But if there were only 10 people in the room, and 6 of them were malicious attackers, those attackers would have the majority—or over 51%—of the voting power.

When malicious attackers are able to take over a cryptocurrency’s network, they can falsify new transactions, prevent valid transactions from being validated, and more.

Generally, these types of attacks are uncommon because it can be prohibitively expensive to try and take over a cryptocurrency’s network.

For example, Bitcoin’s hashrate is measured in hundreds of exahashes per second. An exahash is a one followed by 18 zeros, or 1,000,000,000,000,000,000. If an attacker wanted to take over Bitcoin’s network, they would need to provide more than 51% of Bitcoin’s hashrate. The cost of the specialized mining equipment and electricity needed to do so makes it very prohibitive.