51% attack and its consequences for blockchain

New potential vulnerabilities and exploits emerge alongside the development of new digital systems and technologies. There are no absolutely invulnerable systems: virtually all hardware and software are exposed to digital security risks.

Blockchain is no exception, and one of the main threats it faces is the 51% attack.

What is a 51% attack?

A 51% attack is the main vulnerability of decentralized networks built on the Proof-of-Work (PoW) consensus mechanism. In such an attack, a single node or group of miners seizes complete control over transactions and the generation of new blocks.

The risk of a 51% attack arises from the nature of decentralized blockchain systems, which lack a central authority to control the correct sequence of transactions. Instead, it is determined by the node with the most influence. This means that one node, for example, with overwhelming computational power, can gain control over the entire network.

The blockchains most vulnerable to a 51% attack are those based on Proof-of-Work consensus, such as:

Bitcoin
Ethereum (before The Merge update)
Dogecoin
Bitcoin Cash and other Bitcoin forks (Bitcoin Gold, Bitcoin SV, Bitcoin Diamond, and others)
Litecoin
Monero
Ethereum Classic

Consequences of a 51% attack

One of the main side effects of a 51% attack is "double spending" of coins, also known as "double-spend." Double-spending represents the unauthorized issuance of money, including cryptocurrency. In other words, after a successful 51% attack, attackers would be able to spend the same bitcoin multiple times, causing excessive inflation and, as a result, loss of value.

Other possible consequences of the attack include:

Selfish mining, where the miner controlling the network can block transactions unfavorable to them and release hidden blocks.
Forking — the creation of an alternative blockchain without the consent of other miners in the network.

The greater the share of the blockchain controlled by the attacker, the more severe the consequences. For example, if a miner's computational power significantly exceeds that of others (not just by a few percent), the attacker could manipulate the network's hashrate*, steal funds from sidechain contracts and Layer 2 (L2) solutions, as well as from Lightning Network payment channels.

Despite the alarming consequences, 51% attacks have limitations. For example, attackers cannot cancel current or completed transactions, nor are they likely to change miner rewards.

* Hashrate — the total computational power of all miners in the network.

How to protect against a 51% attack?

One of the few ways to prevent a 51% attack is to increase the number of miner nodes. The more miners in the network, the higher its decentralization, and accordingly, the harder it becomes to carry out an attack. Moreover, greater decentralization reduces the profitability of such attacks.

Another way to reduce the risk is by changing the hashing algorithm, which happened with the Vertcoin blockchain after it had already suffered a 51% attack.

Are there other threats to PoW blockchains besides the 51% attack?

In addition to the existing 51% attack, experts fear the potential threat of quantum attacks. They believe quantum computers could calculate a private key from a public key, which would compromise the entire Bitcoin network and other PoW blockchains.

However, at present, this threat is unlikely. According to scientists, breaking the ECDSA algorithm (used to generate public and private keys) would require at least 317 million qubits. At the same time, the most powerful quantum computers, such as Google's Willow, currently have just over 100 qubits.

Have there been cases of 51% attacks on Bitcoin and other networks?

As of 2025, there have been no successful 51% attacks on the Bitcoin blockchain. However, in 2014, the mining pool GHash.io controlled more than half of Bitcoin's hashrate, which gave it the ability to carry out a 51% attack and raised serious concerns about the security of the blockchain protocol. Ultimately, the pool divided its computational power among several pools and advocated for greater network decentralization.

Today, the possibility of a 51% attack on Bitcoin is complicated, since its hashrate exceeds 1000 EH/s. In other words, carrying out a successful attack would require energy consumption equal to that of a small country, and according to Crypto51, the cost of such an attack would exceed $2.5 million per hour.

According to well-known Bitcoin advocate Andreas Antonopoulos, a 51% attack on Bitcoin makes no sense today, since even governments would find the costs prohibitively high.

Given Bitcoin's high decentralization and massive hashrate, it is doubtful that any miner could seize more than 50% of the hashrate. Nevertheless, the risk remains because miners can combine their pools. According to Mempool.space, the combined share of the two largest mining pools, Foundry USA and AntPool, already exceeds 52% of Bitcoin's total hashrate.

In 2018 and 2020, Bitcoin Gold (a Bitcoin fork) was attacked with a 51% attack, but developers quickly released emergency updates to protect the network.

Between 2019 and 2020, Ethereum Classic (which remained after Ethereum's fork) was repeatedly subjected to 51% attacks. During these, attackers reorganized transactions and carried out double-spending of ETC coins, causing millions of dollars in damage.

In August 2025, a suspected attempt at a 51% attack on the Monero network occurred, following the announcement by IOTA founder's company, Qubic, that it had gained control of more than 51% of the hashrate, coinciding with a blockchain reorganization. The news led to a 7% drop in the altcoin's price. However, analysts noted that block reorganization is common in decentralized networks and is not, by itself, proof of an attack.

How is the 51% attack problem being solved?

One of the solutions to protect against 51% attacks has been the introduction of new consensus mechanisms, such as Proof-of-Stake (PoS) and its variations. In PoS blockchains, validators (instead of miners) lock up a large sum of their cryptocurrency in staking, which they cannot instantly withdraw.

In case of an attack on the blockchain, the validators themselves would suffer losses. This makes attacks on PoS blockchains economically unprofitable for validators while simultaneously incentivizing them to protect the decentralized network. Moreover, PoS blockchains allow restrictions on the maximum staking share, which prevents any single node or group of nodes from gaining excessive influence over the network.