51% Attack

tl;dr: A 51% attack is when an individual or group controls over 50% of the resources of a blockchain network, such as computing power or staked cryptocurrency. It can allow attackers to manipulate transactions, double-spend coins, or modify previously confirmed transactions, but executing such an attack in either a Proof of Work (PoW) or Proof of Stake (PoS) system would be difficult and expensive. While successful 51% attacks have occurred on smaller blockchains, there has not been a successful 51% attack on the Bitcoin network.


What Is a 51% Attack?

A 51% attack is a type of attack on a blockchain network where a single entity or group of entities control more than 50% of the network's resources, which can be either computing power or staked cryptocurrency, depending on the consensus algorithm used by the network.

In a Proof of Work (PoW) system, such as the one used by Bitcoin, a 51% attack occurs when a single entity or group controls more than 50% of the network's computing power, which is also known as the hash rate. With this level of control, the attacker could potentially manipulate transactions, double-spend coins, or modify previously confirmed transactions.

In contrast, in a Proof of Stake (PoS) system, such as the one used by Ethereum, a 51% attack would require the attacker to control more than 50% of the network's total staked Ether. With this level of control, the attacker could potentially manipulate transactions and create new blocks with false transactions.

While a 51% attack is considered a serious threat to the security and integrity of a blockchain network, executing such an attack in either a PoW or PoS system is difficult and expensive, as it requires a significant amount of resources and coordination to gain majority control.

51% Attack(s) On Bitcoin

While there have been successful 51% attacks on significantly smaller blockchains, such as Bitcoin Gold (BTG) [1] and Vertcoin (VTC) [2], there has not been a successful 51% attack on the Bitcoin network so far.

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Do not confuse Bitcoin Gold (BTG) with Bitcoin (BTC). Bitcoin Gold is a hard fork of Bitcoin that operates independently and has its own unique set of rules and characteristics, making it separate from the original Bitcoin network.

In 2014, the mining pool GHash.IO briefly controlled over 50% of the Bitcoin network's hash rate, leading to concerns about the centralization of mining power. However, no attack was carried out, and the incident prompted calls for decentralization to prevent such a concentration of power.

You can find an estimation of the current hash rate distribution among the largest Bitcoin's mining pools on Blockchain.com.

[1] https://forum.bitcoingold.org/t/double-spend-attacks-on-exchanges/...
[2] https://medium.com/.../vertcoin-vtc-is-currently-being-51-attacked-...