What Is a 51% Attack? Bitcoin's Greatest Fear

Cryptocurrencies like Bitcoin, which harness advanced cryptographic techniques to secure their blockchains, are sometimes thought of as “unhackable” or “immutable.” But while blockchains offer powerful security features, they aren’t invincible. A bad actor could theoretically take control of a blockchain using a 51% attack. Here’s how a 51% attack works, what it looks like, and why it probably won’t happen to Bitcoin.

Key Takeaways

A 51% attack happens when a single miner or group controls over 50% of a blockchain network’s total mining power (hashrate).
Attackers can reverse their own transactions, enabling them to spend the same coins twice (“double-spending”), which undermines trust in the network.
While powerful, a 51% attack cannot create new coins or alter past valid transactions by other users. It can only manipulate recent blocks and confirmations.

What Is a 51% Attack?

Blockchain technology relies on a global, decentralized network of computers, known as nodes. When a transaction occurs, each node records it and checks it against the rules to confirm that it is valid. This collective process helps secure the blockchain and makes it resistant to tampering and fraud. One, two, or even 1,000 bad actors can’t just tamper with transactions or add invalid blocks to the Bitcoin blockchain.

However, despite its security features, the blockchain does have one main weakness: the 51% attack, also known as a majority attack. This is when a malicious miner or group of miners gains control of more than half of the blockchain network’s computational power. If this happens, the attacker can manipulate the blockchain and potentially reverse transactions or prevent the confirmation of new ones. While other kinds of networks can also suffer 51% attacks, the process for non-Proof-of-Work blockchains (like Ethereum) would be a bit different.

Such an attack has occurred on smaller networks in the past. For example, Bitcoin Gold, The Verge, and Vertcoin have all suffered at least one 51% attack. However, it’s unlikely that such a large and decentralized blockchain like Bitcoin would suffer an attack of this kind.

Why Would Someone Launch a 51% Attack?

The obvious motive is profit. For example, previous 51% attacks have allowed the attackers to steal money by engaging in “double-spending.” This is when the system is rigged so that the same coins can be spent twice (or more).

For example, an attack on the Bitcoin Gold blockchain in 2018 saw $18 million worth of cryptocurrency stolen as a result of double spending. However, this motive doesn’t make much sense when it comes to large networks as the cost to attain enough computational power (or coins in Proof-of-Stake networks) is so high that it would be essentially impossible to profit from such an attack.

An attack could also be motivated by politics or ideology. A 51% attack would allow attackers to prevent certain transactions from being confirmed. It could also destroy the blockchain network by damaging trust.

Understanding Bitcoin Mining

Transactions are validated and added to the blockchain in groups known as blocks. On some blockchains, like Bitcoin, the validation process is called mining. Blockchains that use mining in this way are called Proof-of-Work (PoW) blockchains.

Mining secures the blockchain by ensuring that every transaction meets the network’s rules. It also helps to prevent tampering and double spending – which is when the same units of currency are spent more than once.

Miners are nodes (computers on the network) that use computational power to solve complex mathematical puzzles. The first miner to solve the puzzle earns the right to add a new block to the chain.

Successful miners are compensated with transaction fees and newly minted cryptocurrency coins – known as the block reward. The block reward for Bitcoin miners is currently set 2.125 BTC per block, but it halves every four years in an event called the halving.

What Is a Mining Pool?

In the early years of Bitcoin, anyone with a computer could, at least in theory, take part in mining. However, as the network grew, the computational resources (or hash power) required increased dramatically, making Bitcoin mining more competitive. Today, Bitcoin mining is often done by specialized companies with large-scale operations and specialist hardware.

To compete with these companies, individual miners usually combine their hash power into a pool by connecting their devices over a network. This improves their chances of solving the puzzle and earning the reward. The miners split any rewards that they earn proportionally. Pooling services facilitate this practice and often charge miners a fee to participate.

Mining pools are not unique to Bitcoin. Other PoW chains like Vertcoin also have mining pools.

Is Bitcoin Mining Pool Centralization a Threat?

The whole idea behind Bitcoin is a decentralized financial system, free from the control of a central entity. Some people worry that mining pools concentrate too much power in too few hands. If a mining pool or group of mining pools were to gain control of the majority of Bitcoin’s total hash power, they could, theoretically, carry out at 51% attack.

It’s true that the major mining pools command a large proportion of the network. In March 2025, three pools – Foundry, Antpool and ViaBTC – collectively controlled 65% of the Bitcoin’s hashrate or computational power. In fact, Foundry USA recently claimed 40% of the global hashrate on its own.

Bitcoin mining pool distribution chart | Source: Blockchain.com

Despite the increasing power of pools, a 51% attack on the Bitcoin network is still extremely unlikely.

Why a 51% Attack Probably Won’t Happen Soon

Majority attacks post a genuine threat to smaller networks, including many altcoins, but an attack on Bitcoin or any large network probably won’t happen. Here’s why.

1. Cost & Computational Power

An attacker would need an enormous amount of computing power to gain majority control of a large network like Bitcoin – and maintain that control. One estimate puts the cost of a 51% attack at $20 billion. Even if a miner gained control of 51% of the network, it would probably be prohibitively expensive to use that power to do things like reversing transactions.

Another reason why this is so infeasible is that the attacker would need a massive amount of the specialized hardware called application-specific integrated circuits (ASICs) that can do nothing but mine Bitcoin. If Bitcoin crashes, these Bitcoin miners would be worth vastly less (if anything), causing even greater losses for the attacker.

ASIC miners — ASIC Bitcoin miners | Source: Bitcoin.com

As a result, 51% attacks have previously occurred only on smaller blockchain networks, like Bitcoin Gold, which can be attacked with far less money and computing power.

2. Incentives

Once the Bitcoin network is attacked, the price of Bitcoin would almost certainly plummet, making it likely that the attackers (e.g., pool operators) would lose potentially billions of dollars rather than making a profit. Both individual miners and pool operators rely on the integrity of the network, and a 51% attack would disrupt and discredit the network. Miners are therefore not incentivized to support a 51% attack and would likely abandon the pool, reducing the pool’s computing power below the necessary threshold.

3. Safeguards

In the case of a 51% attack, the Bitcoin network is designed to adjust. The difficultly of mining would increase, making it harder for the attacker to maintain control. Hard forks and protocol changes are other mechanisms that would come into play to protect the network.

What Would a 51% Attack on Bitcoin Look Like?

In the unlikely event that the Bitcoin network suffered a successful 51% attack, the attackers would be able to:

Steal from others by double-spending. This is where the same coins are sent to two people, leaving one of them out of pocket.
Halt payments by preventing new blocks from being added to the blockchain network.
Reverse recent transactions.
Block the network, making it inaccessible to other users.

However, an attacker’s power would be limited. They would not be able to steal Bitcoin directly from another user or transact from another user’s address. Nevertheless, they would likely be able to reverse fully confirmed transactions, create new coins, or change the underlying Bitcoin protocol.

Impact on the Bitcoin Community

An attack would disrupt activity on the network and undermine user confidence, likely leading to a crash in the price of Bitcoin. This could cause financial devastation for investors and traders who hold a lot of Bitcoin. Miners may pull out of mining amidst the uncertainty, and lose out on the related income as well as the diminished value of the mining hardware. Merchants that rely on timely Bitcoin transactions could lose out on business because of slow payment processing.

Network participants would likely choose to abandon the network in droves. Investors may sell their Bitcoin and invest in another cryptocurrency or move away from cryptocurrency altogether.

How Would Bitcoin React to a 51% Attack?

The community and developers would react fast to counter the attack. They would make a public announcement about the attack and increase monitoring of the network. Bitcoin would also automatically increase mining difficulty to steepen the computing power needed to mine it. This would make it challenging for the attacker to maintain control.

As a last resort, the community may also consider a hard fork. This is a permanent change to the blockchain’s protocol that creates a new version of the blockchain that is incompatible with the previous version. It’s a way of reversing the effects of an attack and upgrading security features. For example, Ethereum instituted a hard fork after a colossal attack in 2016. However, a hard fork is extremely controversial and can lead to community division, loss of trust in the blockchain, and price volatility.

Conclusion

While a 51% attack on the Bitcoin blockchain is theoretically possible, it’s highly unlikely due to the enormous computing power required and well-designed incentives. Should one occur, the community and the Bitcoin protocol itself would quickly respond, fighting to protect its integrity. Loss of trust in the network and the subsequent decrease in Bitcoin’s value would almost certainly leave the attackers at a major loss. Cryptocurrency investors who are concerned about such attacks might want to avoid smaller networks that are more vulnerable.

FAQs

Has Bitcoin ever had a 51% attack?

No. While some mining pools have come close to controlling more than 50% of the network hashrate, there has never been a successful 51% attack on the Bitcoin network.

Why is double spending bad?

Double spending undermines trust in a currency by allowing the same coins to be spent twice. When double spending takes place, one of the two transactions is canceled, leaving one recipient unpaid. This causes financial loss for the recipient and creates mistrust in the systems that keep track of where currency goes.

Can a 51% attack permanently destroy Bitcoin?

No, a 51% attack cannot permanently destroy Bitcoin. While it could disrupt the network, damage trust in Bitcoin, and cause price volatility, the underlying technology and community resilience make a full collapse unlikely.