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What Is a 51% Attack? Meaning, Cost, and Examples

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Ihssan El Medkouri

Author

Ihssan El Medkouri

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Ihssan is a dedicated Content Editor at Cryptonews, where she excels in producing well-researched, insightful content. She has made notable contributions to the cryptocurrency industry, including...

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Last updated:

April 23, 2025

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A 51% attack happens when a single person or group controls over half of a blockchain network’s computing power. With this level of control, they can block transactions, reverse them, and even double-spend coins.

It represents a big threat to the security and trust of decentralized systems. While larger networks like Bitcoin and Ethereum are better protected due to widespread mining power, smaller blockchains with fewer participants face a greater risk.

Key Takeaways

Both Proof-of-Work (PoW) and Proof-of-Stake (PoS) blockchains can be vulnerable to 51% attacks, but the attack method differs for each system.
Successful 51% attacks enable attackers to reverse transactions and double-spend coins, which can damage trust in the network.
The cost of a 51% attack varies widely; it’s generally too expensive on large networks but feasible on smaller or newer blockchains.
Preventing 51% attacks often involves enhancing network decentralization, security protocols, and community participation.
Real-world examples like the Bitcoin Gold and Ethereum Classic attacks illustrate the potential consequences for crypto prices and security.

How Does a 51% Attack Work?

In a 51% attack, an attacker gains control of over half the computational power in a blockchain network. With this majority, they can disrupt the network’s normal operation in two major ways:

Blocking new transactions: The attacker can prevent other users’ transactions from being added to the blockchain, creating delays and essentially freezing the network for others.
Reversing their transactions: This allows the attacker to double-spend, spending the same coins more than once by rolling back transactions.

While a 51% attack lets an attacker modify transaction data, it has limits. For example, they can’t create new crypto coins or access unrelated wallets. The risk is mainly in altering the network’s integrity and disrupting security.

51% Attacks in Proof-of-Work (PoW) Networks

In Proof-of-Work (PoW) networks, miners solve complex puzzles to add new blocks, securing the blockchain through decentralized computing.

To carry out a 51% attack, a person or group must hold the majority of mining power to manipulate transaction data by controlling the block verification process. Networks with more computing power, like Bitcoin, are generally safer because they require vast resources to attack.

On smaller PoW blockchains, attackers could reach 51% more easily and potentially double-spend or block new transactions. This vulnerability makes decentralization and strong security critical for mining in PoW networks.

51% Attacks in Proof-of-Stake (PoS) Networks

In Proof-of-Stake (PoS) networks, validators earn the right to add blocks based on their staked tokens rather than computational power.

Here, a 51% attack requires gaining control of 51% of the staked tokens. Achieving this is costly and impractical since it would require a massive investment and could devalue the tokens used for the attack.

For example, when Ethereum transitioned to PoS, it became more economically secure as the cost of accumulating a majority stake increased. This model creates a safer system by making attacks financially draining and less feasible.

However, PoS networks still rely on decentralized staking to maintain integrity, with blockchain validators as essential parts of security.

Consequences of a Successful 51% Attack

When a 51% attack succeeds, the impact goes beyond just manipulating transactions. It disrupts the blockchain’s security and can deeply shake user confidence. Below are the main consequences of such an attack.

Transaction Blocking and Reversal

In a 51% attack, attackers can block transactions from being validated, effectively freezing other users’ activity. By controlling transaction processing, they can reverse transactions to enable double-spending, using the same coins multiple times.

This control opens the door to potential Denial-of-Service (DoS) attacks, where attackers flood the network, preventing honest participants from accessing the system as their transactions are left in the mempool.

With control over crypto nodes, the attacker can maintain this interference, disrupting the network’s natural flow and reducing user reliability.

Impact on Blockchain Security and Trust

Successful 51% attacks have severe effects on crypto security and trust. When users see that a network can be manipulated, their confidence in that blockchain and the value of its coin drops.

This erosion of trust can lead to long-term consequences, affecting user adoption and cryptocurrency prices, as seen in multiple cases. Rebuilding trust after a successful attack is challenging, and the coin may never fully recover its reputation or market value.

Examples of 51% Attacks

A couple of attacks left a mark on the crypto world and proved the vulnerability of some networks and the need for more secure systems. For example:

Bitcoin Gold (BTG) Attack in 2018

In 2018, Bitcoin Gold (BTG) faced a 51% attack where attackers successfully double-spent transactions, resulting in substantial financial losses. This attack demonstrated the vulnerability of smaller networks to major manipulation, which led to a decline in BTG’s reputation and value.

Ethereum Classic (ETC) Attack in 2020

Ethereum Classic (ETC) faced multiple 51% attacks in 2020. Exploiting the network’s lower hashing power, attackers manipulated transactions, raising concerns about ETC’s security and causing a lasting impact on its trustworthiness in the crypto community.

Ethereum Classic (ETC) Attack (August 2024)

In 2024, the Ethereum Classic network was again targeted. The attack led to multiple double-spending incidents and transaction disruptions, causing immediate financial harm and damaging ETC’s reputation.

These cases show the critical role of decentralization in securing networks, particularly smaller ones. For a blockchain to maintain user trust and stable token prices, it must ensure that a single entity cannot gain majority control.

They also highlight the importance of educating the community about these risks and exploring ways to strengthen security without compromising decentralization.

51% Attack Costs

The resources required to pull off a 51% attack vary significantly depending on the blockchain’s size and consensus mechanism. In PoW systems like Bitcoin, an attacker needs substantial hash power to overtake 50% of the network’s mining capacity.

For PoS systems, attackers need enough capital to own or control 51% of staked tokens. This makes large networks with high participation rates incredibly costly to attack, while smaller, less secure networks are more vulnerable.

Here’s a breakdown of the estimated costs of a 51% attack on various well-known PoW blockchains to provide a clearer picture. Costs fluctuate with changes in network difficulty and market rates for mining equipment and electricity, so these values serve as rough estimates

Network Name	Symbol	Hourly Estimated 51% Attack Cost
Bitcoin Cash	BCH	$7,594
Litecoin	LTC	$94,232
Ethereum Classic	ETC	$9,319
eCash	XEC	$449
Zcash	ZEC	$4,173
Ravencoin	RVN	$2,403

Challenges in Executing a 51% Attack

Although a 51% attack is technically possible, executing one successfully poses several challenges that make it difficult to carry out:

➡️ Perfect Timing Required

Attackers must precisely time their moves to maximize disruption. Any miscalculation can alert the network and prompt countermeasures, undermining the attack’s effectiveness.

➡️ High Costs for PoW Attacks

Launching an attack on a PoW network is resource-intensive. To overtake 51% of the network’s computing power, an attacker would need massive investments in mining hardware and electricity.

The operational costs quickly increase, especially on bigger networks, where securing the necessary hash power requires substantial capital and a stable energy source.

➡️ Financial Implications for PoS Attacks

In a PoS network, attackers must acquire many staked tokens. However, PoS systems often include penalties for malicious activity, known as “slashing.”

If caught, an attacker risks heavy losses through slashed stakes and potential collateral loss, which adds another layer of risk and cost to their efforts.

Prevention and Mitigation of 51% Attacks

Preventing 51% attacks requires strategies that boost network resilience through decentralization, participation, and security measures. Let’s discuss some key factors that can prevent this type of attack.

Decentralization

A robust, decentralized network is one of the most effective defenses against 51% attacks. When a blockchain has many independent nodes and participants, it’s much harder for a single entity to control more than half of the network.

This wide distribution of computing or staking power adds layers of security and minimizes the risk of one party overpowering the network.

Network Size and Participation

The larger and more active a network, the more challenging it becomes to launch a 51% attack. Major networks like Bitcoin and Ethereum have vast computational resources (or staked assets in the case of PoS systems), making it almost impossible for a single group to amass the necessary resources for an attack.

High participation rates and computational requirements act as a natural deterrent, reducing vulnerabilities associated with centralization.

Consensus Mechanism and Mitigation Techniques

Consensus models like PoS incorporate in-built protections such as slashing to penalize malicious behavior, adding extra financial risk for potential attackers.

Beyond consensus mechanisms, other mitigation strategies are often in place:

Slashing Mechanisms in PoS: Bad actors risk losing their staked tokens, which makes it financially discouraging even to attempt an attack.
Regular Audits and Network Monitoring: By closely monitoring activity and conducting regular audits, networks can quickly detect and respond to suspicious actions.
Delaying Blockchain Confirmations: Extending confirmation times for transactions allows the network more time to verify data, limiting the scope of a potential attack.

Emerging Solutions

Emerging defenses against 51% attacks are getting smarter on both PoW and PoS chains. For PoW, ghost-based protocols like GHOSTDAG reduce the advantage of chain reorgs by leveraging a DAG structure, making attacks less profitable. Some networks also use manual or decentralized checkpointing to lock in history past certain depths.

Meanwhile, PoS networks are exploring slashing conditions that penalize validators for signing conflicting blocks, combined with finality gadgets like Ethereum’s Casper FFG that make finalized blocks economically irreversible. Some projects are experimenting with hybrid models, combining PoW and PoS, to mitigate their weaknesses.

Another promising angle is social consensus: even if an attacker rewrites history, clients may ignore it if the community rejects the fork. While not purely technical, this adds a powerful layer of defense. Ultimately, the direction is clear — raising the attack cost while reducing potential gains, through protocol incentives and cryptoeconomic enforcement.

Final Thoughts on 51% Attacks

While a successful 51% attack remains unlikely on large, decentralized networks, smaller networks can be more susceptible, highlighting the importance of ongoing security enhancements. Blockchain developers and participants should focus on implementing and supporting robust defenses to uphold network integrity.

Explore blockchain encryption, the backbone of data security in these systems, to deepen your understanding of blockchain security.

👉 Learn More: What Is Blockchain Encryption?