51% Attack: What Is It, How Does It Work, and Who Is At Risk

ELI5 Definition
Last updated: Jul 28, 2023

Understanding the 51% Attack in Blockchain Networks

In the realm of blockchain technology, security is of paramount importance to ensure the integrity and immutability of distributed ledgers. One of the most concerning threats to blockchain networks is the dreaded 51% attack, a malicious scenario that can compromise the underlying principles of decentralization and trust in such systems.

💡 Key Ideas

  • Definition: The 51% attack is a malicious scenario where an individual or group controls more than 50% of a blockchain network's hash rate, granting them disproportionate influence over the network's operations.

  • Mechanics: The attackers create a longer, private fork of the blockchain in secret, enabling them to execute conflicting transactions on the public chain and perform double-spending exploits.

  • Consequences: Successful 51% attacks can lead to double-spending, loss of trust, network manipulation, and raise concerns about centralization, impacting the network's reputation and future participation.

  • Preventive Measures: To mitigate the risk of 51% attacks, blockchain networks can promote increased hash rate distribution, explore alternative consensus protocols, continuously monitor network activity, and advocate for decentralization.

What is a 51% Attack?

A 51% attack, also known as a majority attack or double-spending attack, refers to a situation in which a malicious actor or a group of colluding actors gain control over more than 50% of the total computational power (hash rate) in a blockchain network. This nefarious majority allows them to exert disproportionate influence over the network's operations and manipulate its transactions.

The Mechanics of a 51% Attack

To understand the mechanics of a 51% attack, let's consider a simplified example using a hypothetical blockchain network:

  1. Consensus Protocol: Most blockchain networks, such as Bitcoin and Ethereum, employ a consensus protocol, like Proof of Work (PoW). In PoW, miners compete to solve complex cryptographic puzzles to validate and add new blocks to the blockchain. The first miner to solve the puzzle gets the right to add the block and is rewarded with newly minted cryptocurrency.

  2. Block Confirmation: Once a block is added to the blockchain, it needs a certain number of subsequent blocks to be appended on top of it to achieve a higher level of security and immutability. This process is known as block confirmation.

  3. The Attack: In a 51% attack, the malicious entity or group controls more than 50% of the network's hash rate. They can use this power to extend their private blockchain, creating a longer chain in secret that contains different transactions. This chain is known as a fork.

  4. Double Spending: By creating a fork, the attackers can now perform a double-spending attack. In this scenario, they initiate a transaction on the public chain, say sending cryptocurrency to an exchange, while simultaneously initiating a conflicting transaction on their private fork, like transferring the same cryptocurrency to another address they control.

  5. Fork Reorganization: To succeed, the malicious actors must keep their longer fork a secret until it surpasses the length of the public chain. Once achieved, they release the private chain to the network. This causes a reorganization of the blockchain, and the network accepts the longer fork as the valid chain, effectively erasing the original transaction on the public chain. Consequently, the attackers' double-spending transaction becomes the legitimate one.

Impact and Consequences

The 51% attack poses significant risks and consequences for blockchain networks:

  1. Double Spending: The most immediate consequence is the ability to double-spend cryptocurrencies, undermining the network's integrity and causing financial losses.

  2. Loss of Trust: A successful 51% attack can lead to a loss of confidence in the blockchain network, deterring users, investors, and businesses from participating.

  3. Centralization Concerns: It raises concerns about centralization, as the attack undermines the decentralization principle that blockchain networks aim to achieve.

  4. Network Manipulation: The attackers could censor transactions, prevent certain transactions from being confirmed, or even reverse legitimate transactions.

  5. Economic Incentive: The potential economic rewards from a successful 51% attack might incentivize further attacks on vulnerable networks.

Who is at Risk?

The risk of a 51% attack in a blockchain network primarily depends on the consensus mechanism employed by the network. Networks that utilize Proof of Work (PoW) as their consensus protocol are particularly vulnerable to this type of attack. PoW blockchains, like Bitcoin and Ethereum (pre-Ethereum 2.0 transition), rely on miners to compete and solve complex mathematical puzzles to add new blocks to the chain.

Miner Concentration: In PoW systems, the risk of a 51% attack is higher when a significant portion of the network's hash rate is controlled by a small number of mining entities. A concentration of mining power in the hands of a few miners or mining pools increases the potential for collusion and the formation of a malicious majority.

Less Popular Networks: Smaller and less popular blockchain networks with lower hash rates are more susceptible to 51% attacks. These networks might not attract enough miners, making it relatively easier for attackers to accumulate the required hash rate for a successful attack.

Attackers' Motivation: The risk also depends on the economic incentives for attackers. If the potential rewards from a successful attack outweigh the costs, attackers may be more motivated to target a particular blockchain network.

On the other hand, blockchain networks that use alternative consensus mechanisms like Proof of Stake (PoS), Delegated Proof of Stake (DPoS), or Proof of Authority (PoA) are less prone to 51% attacks. These protocols use different mechanisms for block validation, and their security models are designed to mitigate the risk of a malicious majority gaining control.

Despite the risk, it is important to note that successful 51% attacks are relatively rare, as they often require substantial resources, planning, and technical capabilities. Nevertheless, understanding the risk and taking appropriate preventive measures remains essential to ensure the security and stability of blockchain networks.

Preventive Measures

While a 51% attack is a concerning possibility, blockchain developers and stakeholders can adopt various preventive measures:

  1. Increased Hash Rate: Encouraging more participants to join the network as miners can increase the overall hash rate, making it more challenging for malicious actors to control a majority.

  2. Consensus Protocol Improvements: Developers can explore alternative consensus protocols like Proof of Stake (PoS), Delegated Proof of Stake (DPoS), or Proof of Authority (PoA), which have different security models and are less susceptible to 51% attacks.

  3. Network Monitoring: Continuous monitoring of network hash rate distribution can help detect and respond to suspicious fluctuations.

  4. Decentralization Advocacy: Promoting decentralization principles within the blockchain community can help maintain a distributed and resilient network.

Real-World Example: The Ethereum Classic 51% Attack

One of the most notable real-world examples of a 51% attack occurred on the Ethereum Classic (ETC) blockchain in early 2019. Ethereum Classic is a prominent cryptocurrency that emerged as a result of a hard fork from the original Ethereum network following the infamous DAO (Decentralized Autonomous Organization) hack in 2016.

The Attack

In January 2019, the Ethereum Classic network fell victim to a 51% attack, which had significant implications for the blockchain's security and reputation. During the attack, the malicious actor(s) managed to gain control of more than 50% of the network's hash rate, granting them unprecedented control over the blockchain.

With this majority control, the attackers proceeded to create an alternative, longer fork of the Ethereum Classic blockchain in secret. They then initiated transactions on their private fork while simultaneously depositing an equivalent amount of ETC to cryptocurrency exchanges.

Double-Spending Exploitation

The primary objective of the attackers was to exploit the double-spending vulnerability in the network. Using their longer, private fork, they made transactions on the public Ethereum Classic blockchain and, at the same time, executed conflicting transactions on their hidden fork.

For instance, the attackers could deposit a certain amount of ETC to a cryptocurrency exchange on the public blockchain while simultaneously initiating a transaction on their private fork, sending the same ETC to another address they controlled. Since they held a majority of the network's hash rate, their secret fork could grow faster than the public chain.

Reorganization and Double-Spending

Once the attackers' secret fork became longer than the main chain, they orchestrated a "reorganization" of the Ethereum Classic blockchain. They released their private chain to the public network, replacing the original chain.

As a result, all the transactions and blocks on the public blockchain that were created after the point of the 51% attack were effectively reversed. This included the legitimate transaction where the attackers had sent ETC to the exchange. Instead, the ETC they had sent to another address on their private fork was now considered the valid transaction. The outcome: the attackers successfully double-spent their ETC and withdrew funds from the cryptocurrency exchange.

Impact and Aftermath

The Ethereum Classic 51% attack raised serious concerns about the security and robustness of blockchain networks that rely on Proof of Work consensus. The attack led to a loss of confidence in the Ethereum Classic network, with users and investors questioning its ability to safeguard against future attacks.

The incident also sparked debates within the blockchain community about the viability of Proof of Work as a consensus mechanism, and it prompted Ethereum Classic developers to explore alternatives, including a potential transition to Proof of Stake.

Lessons Learned

The Ethereum Classic 51% attack serves as a crucial real-world example that highlights the importance of network security and the vulnerabilities associated with Proof of Work blockchains. It emphasized the need for continuous monitoring, increased hash rate distribution, and proactive measures to mitigate such attacks.

Furthermore, the incident underscored the importance of a robust and vigilant community that collaboratively works to uphold the principles of decentralization and integrity in blockchain networks. Implementing preventive measures and considering alternative consensus protocols can bolster the security of blockchain networks and safeguard against the threat of 51% attacks.


The 51% attack remains a critical concern for blockchain networks, posing a significant threat to their integrity and security. Understanding the mechanics behind this attack and its potential consequences empowers developers and stakeholders to implement appropriate preventive measures. By fostering a collaborative and vigilant community, blockchain networks can continue to evolve securely, preserving the trust and decentralized ethos they were designed to uphold.

Disclaimer: The information provided in this article is for educational purposes only. No part of it should be considered as financial or investment advice.