Knowledge Center

What Is a 51% Attack?

A 51% attack refers to a blockchain attack by a group of miners who control more than 50% of the network’s mining hash rate. This type of attack is most commonly connected to Proof of Work (PoW) blockchains.

The Bitcoin blockchain is a distributed ledger that records every transaction made on the network. Based on the fact that the blockchain is decentralized, all transactions are available for public viewing. Consequently, this prevents the double-spending of coins because the transaction would quickly be spotted and rejected by the network’s users.

Goal of a 51% Attack

The objective of a 51% attack is to control a majority of the computing power on the network. This usually requires a group of attackers to work together. Although the attackers can’t alter previous transactions, they can interfere with the process of recording new blocks.

For example, they can prevent miners from completing blocks, which allows the attackers to monopolize the mining of new blocks and earn all of the rewards.

If a group of attackers controls more than 50% of the network’s mining hash rate, they can block other users’ transactions. This allows the attackers to send a transaction and then suddenly reverse it, thus making it appear as though they still had possession of the coin.

In reality, the coin has already been spent. This is a classic example of double-spending. The best way to safeguard against double-spending is by preventing a 51% attack from occurring.

Are 51% Attacks Common?

Based on the sheer size of the Bitcoin blockchain, the chance of a 51% attack is highly remote. On smaller blockchains with less hashing power, such as Bitcoin Cash or Ethereum Classic, 51% of attacks have occurred several times.

The likelihood of a single miner (or group of miners) obtaining enough computing power to overwhelm all the other participants on the Bitcoin blockchain is practically non-existent. As the blockchain grows, changing the previously confirmed blocks becomes more difficult and expensive.

This explains why the chances of a 51% attack will continue to diminish as the Bitcoin blockchain grows.

Brief Summary of a 51% Attack

  • A 51% attack refers to a blockchain attack by a group of miners.
  • An attack is possible if miners control more than 50% of the network’s hashing rate.
  • This type of attack is most commonly connected to Proof of Work (PoW) blockchains.
  • The objective is to control a majority of the computing power on the network.
  • The attackers can’t alter previous transactions.
  • However, they can interfere with the process of recording new blocks.
  • Double-spending is a major concern if hackers successfully launch a 51% attack.
  • Based on the sheer size of the Bitcoin blockchain, a 51% attack is highly unlikely.
  • The chances of a 51% attack on the Bitcoin blockchain will continue to diminish as the Bitcoin blockchain grows, however, smaller blockchains may still be at risk.
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