# What is a 51% Attack on Blockchain? Risks, Examples, and Costs Explained

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*— Investopedia / Jessica Olah*

## What Is a 51% Attack?

A 51% attack occurs when an entity gains control over more than 50% of a blockchain network's hashing power, enabling the alteration of transactions and creating the risk of double-spending. While challenging and costly, such attacks primarily threaten smaller networks. Explore the mechanics, examples, and costs associated with 51% attacks.

### Key Takeaways

*   A 51% attack occurs when an entity controls more than half of a blockchain's computing power, potentially altering transactions and blocking new ones.
*   These attacks are mostly a threat to smaller cryptocurrency networks due to lower hash rates and participation.
*   Despite the high cost and difficulty of executing a 51% attack, cloud services can allow attackers to rent necessary computing power, increasing vulnerability.
*   Larger networks like Bitcoin and Ethereum are more secure against 51% attacks due to their size and complexity.
*   If successful, a 51% attack allows for double-spending and denial-of-service, but it requires precise timing and significant resources to execute.

## How 51% Attacks Affect Blockchain Security

A [blockchain](https://www.investopedia.com/terms/b/blockchain.asp) is a distributed ledger—essentially a database—that records transactions and information about them. The blockchain's network reaches a majority consensus about transactions through a validation process. The blocks where the data is stored are sealed. The blocks are linked together via cryptographic techniques where previous block information is recorded in each block. This makes the blocks nearly impossible to alter once they are confirmed enough times.

The 51% attack is an attack on the blockchain, where a group controls more than 50% of the hashing power—the computing that solves the cryptographic puzzle—of the network. This group then introduces an altered blockchain to the network at a very specific point in the blockchain, which is theoretically accepted by the network because the attackers would own most of it.

Changing historical blocks—transactions locked in before the start of the attack—would be extremely difficult even in the event of a 51% attack. The further back the transactions are, the more difficult it is to change them. It would be impossible to change transactions before a checkpoint, where transactions become permanent in Bitcoin's blockchain.

### The High Costs of Launching a 51% Attack

A 51% attack is a very difficult and challenging task on a blockchain network with a large participation rate. In most cases, the group of attackers would need to be able to control the necessary 51% and have created an alternate blockchain that can be inserted at exactly the right moment. Then, they would need to out-hash the main network. The high cost is a key factor that prevents a 51% attack.

For example, the WhatsMiner M63S is an advanced ASIC miner costing over $10,000 and has a hash rate of 406 TH/s. A single or smaller group of miners would not be able to alter and mine the Bitcoin blockchain with only a few of these machines. It would take thousands of these ASICs to get ahead of the Bitcoin network. Smaller networks could be out-hashed using these mining rigs, but the benefits of doing so wouldn't outweigh the costs of funding the attack and setting it up.

> **Fast Fact**
>
> Hashing power rental services provide attackers with lower costs, as they only need to rent as much hashing power as they need for the duration of the attack.

After Ethereum transitioned to proof-of-stake, a 51% attack on the Ethereum blockchain became even more expensive. To conduct this attack, a user or group would need to own 51% of the staked ETH on the network. It is possible for someone to own that much ETH, but it's unlikely.

According to Beaconchain, more than 32.3 million ETH were staked on May 8, 2024. An entity would need to own and stake more than 16.5 million ETH (more than $49 billion as of May 8, 2024) to attempt an attack.

If an attack begins, the consensus mechanism would likely spot it and slash the staked ETH, costing attackers a lot. Additionally, the community can vote to restore the "honest" chain, so an attacker would lose all of their ETH just to see the damage repaired.

### The Crucial Timing of a 51% Attack

In addition to the costs, a group attempting to attack the network using a 51% attack must not only control 51% of the network but also introduce the altered blockchain at a very precise time. Even if they own 51% of the network hashing rate, they still might not be able to keep up with the block creation rate or get their chain inserted before valid new blocks are created by the 'honest' blockchain network.

This is possible on smaller networks due to fewer participants and lower hash rates. Large networks make introducing an altered blockchain almost impossible.

> **Fast Fact**
>
> You don't need 51% of a network's mining power to attack it, but without it, success is unlikely.

### Consequences of a Successful 51% Attack

In the event of a successful attack, the attackers could block other users' transactions or reverse them and spend the same cryptocurrency again. This vulnerability, known as [double-spending](https://www.investopedia.com/terms/d/doublespending.asp), is the digital equivalent of a perfect counterfeit. It is also the basic cryptographic hurdle blockchain consensus mechanisms were designed to overcome.

Successful 51% attackers may also implement a Denial-of-Service (DoS) attack, where they block the addresses of other miners for the period they control the network. This keeps the "honest" miners from reacquiring control of the network before the dishonest chain becomes permanent.  

## Which Blockchain Networks Face a 51% Attack Risk? 

Mining equipment affects vulnerability. [ASIC](https://www.investopedia.com/terms/a/asic.asp)\-secured networks are less at risk than GPU-mined networks because ASICs are faster. Cloud services such as NiceHash—which considers itself a "hash-power broker"—theoretically make it possible to launch a 51% attack using only rented hash power, especially against smaller, GPU-only networks.

Bitcoin Gold has been a common target for attackers because it is a smaller cryptocurrency by hashrate. Since June 2019, the Michigan Institute for Technology's Digital Currency Initiative has detected, observed, or been notified of more than 40 51% attacks—also called chain reorganizations, or reorgs—on Bitcoin Gold, Litecoin, and other smaller cryptocurrencies.

## Is Bitcoin Likely to Face a 51% Attack? 

On May 8, 2024, the Bitcoin network's total hashrate was 569.29 exahashes per second (EH/s). The top three [mining pools](https://www.investopedia.com/terms/m/mining-pool.asp) by three-day hashrate were:

*   FoundryUSA, at 175.76 EH/s; 30.9% of the total Bitcoin network hashrate
*   AntPool, at 161.77 EH/s; 28.4% of the total Bitcoin network hashrate
*   ViaBTC, at 73.11 EH/s; 12.8% of the total network hashrate

Combined, these three pools made up 72.1% of the network hashrate, a whopping 486.9 EH/s (486.9 million TH/s—the CPU in your computer might be able to hash at about 15 kilo hashes per second). If Foundry and ViaBTC were to collude, they could take over 51% of the hashrate (248 EH/s).

Foundry and Antpool together could control 69.3% of the network. Managers at these pools could order members to work on altered chains. The pool's miners would have no idea which chain they were working on since their mining rigs automatically work on whatever task they are given.

Even more concerning is that these three pools also monopolize the majority of the network hash rates for Bitcoin Cash, Litecoin, and Bitcoin SV.

These pools have been operating for several years without issue, but the fact remains that they already control most hashing power of the minable and profitable cryptocurrencies.  

## What Does a 51% Attack Do?

A 51% attack alters blocks that are being added to the blockchain, giving the attackers the ability to create or alter transactions for the period they are in control.

## Has a 51% Attack Ever Happened?

Yes. Several blockchains have been attacked using this method, but they had small networks, were new, or had other vulnerabilities that made it possible.

## How Much Would a 51% Attack on BTC Cost?

If a large mining pool was directed by its managers to conduct an attack, it wouldn't cost the managers much at the time of the attack. However, it would likely lose its honest miners once they found out about it. For a single person or group to conduct a 51% attack, they would need more than 304 EH/s of computing power. This is an enormous cost considering the fastest miner hashes 406 TH/s and costs more than $10,000 per unit (about 84,000 units).

## The Bottom Line

A 51% attack occurs when a group gains control over more than half of a cryptocurrency network's hashing power, allowing them to alter transactions and engage in double-spending. While these attacks pose a significant threat to smaller networks, they are costly and challenging to execute on larger, more secure networks like Bitcoin and Ethereum. Understanding the mechanics and risks of 51% attacks is crucial for cryptocurrency users, as specific networks remain vulnerable, and users must be aware of potential risks.

_The comments, opinions, and analyses expressed on Investopedia are for informational purposes only. Read our [warranty and liability disclaimer](https://www.investopedia.com/legal-4768893#toc-warranty-and-liability-disclaimer) for more info._  

> **Article Sources**
>
> 1.  O'Reilly. "[Mastering Bitcoin: Chapter 8. Mining and Consensus](https://www.oreilly.com/library/view/mastering-bitcoin/9781491902639/ch08.html#consensus_attacks)."
>     
> 2.  MicroBT. "[WhatsMiner M63S](https://shop.whatsminer.com/products/details/67?skuId=132)."
>     
> 3.  Michigan Institute for Technology Digital Currency Initiative. "[51% Attacks](https://dci.mit.edu/51-attacks)."
>     
> 4.  Beaconchain. "[Open Source Ethereum Explorer](https://beaconcha.in/)."
>     
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>     
> 6.  Ethereum. "[Proof-of-Stake](https://ethereum.org/en/developers/docs/consensus-mechanisms/pos/#pos-and-security)."
>     
> 7.  NiceHash. "[Official Statement Regarding the Latest 51% Attack Allegations](https://www.nicehash.com/blog/post/official-statement-regarding-the-latest-51-percent-attack-allegations-aug-2020)."
>     
> 8.  BTC.com. "[Pool Distribution](https://btc.com/stats/pool)."