A 51% attack is considered one of the most serious threats to blockchains based on the Proof-of-Work (PoW) consensus mechanism. In such an attack, a single miner or a group of miners gains control over more than half of the network's computational power, enabling them to manipulate transactions and block generation.
This risk arises due to the decentralized nature of blockchain systems, where no central authority controls transaction validation. Instead, the dominant node in the network, if it has significant computational power, can control the entire network.
Blockchains most vulnerable to a 51% attack include those based on the PoW consensus mechanism:
- Bitcoin
- Ethereum (before the update, The Merge)
- Dogecoin
- Bitcoin Cash and other Bitcoin forks (Bitcoin Gold, Bitcoin SV, and others)
- Litecoin
- Monero
- Ethereum Classic
Consequences of a 51% attack
The primary consequence of a 51% attack is double-spending, where attackers can spend the same cryptocurrency multiple times, leading to inflation and a decrease in its value.
Other potential consequences of such an attack include:
- Selfish Mining: Miners can block transactions that aren't beneficial to them and delay the creation of blocks.
- Forking: Creation of an alternative blockchain without the consent of other network participants.
The more mining power the attacker controls, the more severe the consequences. For example, suppose a miner's computational power far exceeds others. In that case, they can manipulate the network's hash rate and even steal funds from smart contracts, sidechains, second-layer solutions (L2), and Lightning Network payment channels.
Despite the serious consequences, a 51% attack has its limitations. For example, attackers cannot reverse completed transactions.
How to protect against a 51% attack
One of the most effective ways to protect against a 51% attack is by increasing the number of mining nodes in the network. The more nodes (or miners) participating in the block mining process, the harder it becomes to take control of more than half of the network's computational power.
When many nodes are distributed across multiple geographic regions and controlled by different participants, it becomes significantly more difficult for an attacker to maintain the network. For a successful 51% attack, attackers not only need technical capabilities but also a dominant share of computational power, which is nearly impossible in a highly decentralized network.
Furthermore, decentralization reduces the profitability of such an attack. The more participants there are, the less likely it is that one miner or group of miners can control a large share of the hash rate*. This makes the attack economically unfeasible, as capturing over 50% of the hash rate would require significantly more resources, making it too costly for potential attackers.
* Hash rate is a measure of computational power used by miners to validate transactions and create new blocks on the blockchain. It is measured in hashes per second and indicates how many calculations the network or miner can perform in a given time.
Other threats to PoW blockchains
Apart from the threat of a 51% attack, experts are also concerned about the possibility of quantum attacks. They believe quantum computers could calculate a private key from the public key, which would compromise the entire Bitcoin network and other PoW blockchains.
However, this threat is unlikely for now, as it would take at least 317 million qubits to break the ECDSA algorithm used for creating public and private keys. Currently, the most powerful quantum computers, such as Google's Willow, have just over 100 qubits*.
* A qubit is the basic unit of quantum information in quantum computers. Unlike traditional bits, which can only be in one of two states (0 or 1), a qubit can exist in multiple states simultaneously, enabling quantum computers to perform complex calculations much faster than classical computers.
Have there been 51% attacks on Bitcoin and other networks?
As of 2025, Bitcoin has not experienced a successful 51% attack. However, in 2014, the GHash.io mining pool controlled more than half of Bitcoin's hash rate, which posed a threat of an attack. In response, the pool split its power across multiple pools and called for improved decentralization of the network.
Today, executing a 51% attack on Bitcoin is extremely difficult due to its hash rate exceeding 1000 EH/s. To conduct a successful attack, it would require more energy than a small country could provide, and the cost would exceed $2.5 million per hour.
Bitcoin supporter Andreas Antonopoulos argues that a 51% attack on Bitcoin is economically meaningless today, even for governments.
However, despite its high level of decentralization, the risk of an attack remains, as large pools like Foundry USA and AntPool control more than 52% of Bitcoin's hash rate.
Bitcoin Gold experienced 51% attacks in 2018 and 2020, but after an emergency update, the attacks were prevented.
From 2019 to 2020, Ethereum Classic was repeatedly attacked by 51% attackers, who reorganized transactions and conducted double-spending of ETC, resulting in millions of dollars in losses.
How are 51% attacks addressed?
One of the solutions to protect against a 51% attack is the introduction of new consensus mechanisms, such as Proof-of-Stake (PoS) and its analogs. In PoS blockchains, cryptocurrencies are mined by validators rather than miners. Validators lock a significant amount of their own funds in staking and cannot withdraw them immediately.
If a PoS blockchain is attacked, the validators themselves will suffer the losses. This makes such attacks economically unfeasible for validators and motivates them to protect the network.
In addition, PoS blockchains allow for limiting the maximum share in staking, protecting the network from excessive influence by a single node or group of nodes.