A 51% attack is a type of attack on a blockchain network in which a single entity or group of entities controls more than 50% of the network’s mining hash rate, giving them the ability to manipulate the blockchain. With this level of control, an attacker can prevent new transactions from being confirmed, reverse transactions that have already been confirmed, and double-spend coins.

Blockchains that use a proof-of-work (PoW) consensus mechanisms, such as Bitcoin and Ethereum, are at risk of 51% attacks. Smaller and less secure networks are particularly vulnerable to these attacks.

An example of a 51% attack occurred in January 2019 on the Ethereum Classic blockchain, where an attacker was able to double-spend over $1 million worth of ETC.

The cost of a 51% attack can vary depending on the specific blockchain and the current market value of the cryptocurrency. The cost of renting enough hashing power to perform the attack can be high, but the potential financial gain for the attacker can also be significant. Additionally, a successful 51% attack can damage the reputation and value of the affected blockchain and its associated cryptocurrency.

Attacks Are excessively costly

A 51% attack on a cryptocurrency network is a complex and demanding task that requires a significant amount of resources. The attackers must have control over at least 51% of the network’s hashrate and have a competing blockchain ready to be inserted at the appropriate time. They must also have the ability to out-hash the main network. The high cost of obtaining this level of control is one of the main deterrents to a 51% attack.

For example, the most powerful ASIC miner currently available is the Bitmain S19 XP Hydro, which costs over $19,800 and has a hash rate of 255 terahashes per second (TH/s).

The top three mining pools by hashrate are:

  • FoundryUSA, at 54.42 exahashes per second (EH/s); 23.75% of the total Bitcoin network hashrate
  • AntPool, at 41.49 EH/s; 18.12% of the total Bitcoin network hashrate
  • Binance Pool, at 34.48 EH/s; 15.06% of the total network hashrate

These pools control a significant portion of the total hashrate of the Bitcoin network, making it more difficult and costly to carry out a 51% attack.

The combined hashrate of the top three mining pools, FoundryUSA, AntPool, and Binance Pool, makes up 56.93% of the network hashrate, which is equivalent to 130.4 EH/s (1.304 million TH/s). To achieve this level of hashrate, an attacker would need more than 511,373 Bitmain S19 XP Hydro miners, at a fixed cost of nearly $10.13 billion, not including additional expenses such as equipment housing, maintenance, electricity, and cooling.

Due to the enormous cost, 51% attacks on major cryptocurrencies like Bitcoin are unlikely to occur. These types of attacks are more commonly seen on smaller and less secure networks.

With Ethereum’s transition to proof-of-stake, the cost of a 51% attack has become even higher. To successfully conduct an attack, an entity would need to control 51% of the staked ETH on the network. According to Beaconchain, as of September 2022, more than 13.8 million ETH were staked on the Ethereum network, meaning an attacker would need to own more than 6.9 million ETH (worth over $9 billion) to attempt an attack.

Even if an attack were to occur, the consensus mechanism would likely detect it and immediately slash the attacker’s staked ETH, resulting in significant financial losses. Additionally, the community can vote to restore the “honest” chain, so an attacker would lose all of their ETH without achieving their objective.

Attack Timing and Successful Outcome

Conducting a 51% attack on a network requires not only controlling 51% of the network’s hashing power but also timing the introduction of the altered blockchain perfectly. Even with 51% control, it may be difficult to keep up with the block creation rate or insert the altered chain before new blocks are created by the legitimate network. This is more feasible on smaller networks with lower participation and hash rates, but near impossible on larger networks.

A successful 51% attack could allow attackers to block or reverse legitimate transactions and double-spend the same cryptocurrency, known as double-spending. This vulnerability is similar to creating a perfect counterfeit. Blockchain consensus mechanisms are designed to prevent this type of attack. Additionally, attackers may launch a Denial-of-Service (DoS) attack, blocking the addresses of other miners while they control the network, making it difficult for legitimate miners to regain control before the altered chain becomes permanent.

51% Attack FAQs

Who is vulnerable to a 51% attack?

The type of mining equipment used also plays a role, as networks secured by ASICs are less susceptible than those that can be mined using GPUs. Cloud services such as NiceHash, which allows users to rent hash power, can potentially make it easier to launch a 51% attack, particularly against smaller GPU-only networks.

Smaller cryptocurrencies, such as Bitcoin Gold, are more likely to be targeted by attackers. As of June 2019, the Michigan Institute for Technology’s Digital Currency Initiative has recorded over 40 51% attacks on Bitcoin Gold, Litecoin, and other small cryptocurrencies.

What is a 51% attack?

A 51% attack is a malicious effort to restructure a blockchain by actors who control more than 51% of a cryptocurrency’s total hashing or validating power.

Is a 51% attack on Bitcoin possible?

While it is theoretically possible for a well-funded attacker to launch a 51% attack on the Bitcoin blockchain, the cost of acquiring enough hashing power to do so typically makes it infeasible.

How much Bitcoin is needed for a 51% attack?

A 51% attack is not dependent on the amount of Bitcoin held, but rather on control of mining. As of September 22, 2022, attackers would need to control 115 EH/s of hashing power to launch an attack on the Bitcoin blockchain. This would require more than 511,111 of the most powerful ASIC miners, which have a hashrate of 255 TH/s and cost over $10 billion in equipment alone.