A 51% attack is when an attacker controls more than 50% of the mining operations in a Proof-of-Work (PoW) blockchain. Mining power (computing power), or controlling more than 50% of the equity in a proof-of-stake (PoS) blockchain. This majority control allows attackers to manipulate the blockchain's consensus mechanism, leading to various malicious behaviors, such as:

• Double spend: An attacker can reverse a transaction multiple times Spend the same cryptocurrency. They can create a fork on the blockchain and then add blocks to their own chain while also spending the same currency on the original chain. Once their private chain becomes the longest chain, the entire blockchain network will consider it a valid chain, and transactions on the original chain will be invalidated, allowing the attacker to spend their currency again.
• Censorship: With majority control of the blockchain, an attacker can choose to exclude specific transactions from the blockchain, or delay the execution of the transaction. This can be used to manipulate transaction history or prevent specific users from making transactions.
• Block reward manipulation: Attackers may manipulate the block generation process to obtain more block rewards for themselves, or even completely monopolize the rewards.

A successful 51% attack could compromise the integrity and security of a blockchain network and potentially cause its value to plummet. However, launching such an attack is often difficult and expensive, especially on large, mature networks like Bitcoin or Ethereum. The resources required to perform a 51% attack on these networks would be enormous, making it unlikely that an attacker would attempt it. However, smaller and less secure networks may be more vulnerable to this attack.

In January 2019, Ethereum Classic suffered a 51% attack. Ethereum Classic is a blockchain that is a hard fork of the Ethereum network. In this attack, the attacker managed to double-spend approximately 219,500 ETC, worth approximately $1.1 million at the time. Major cryptocurrency exchanges, such as Coinbase, also suspended trading and deposits of ETC due to the attack.

You may be interested in the following:

• What is a consensus mechanism?
• What is a fork?