51% Attacks in Cryptocurrencies: Definition, Implications, and Safeguards


A 51% attack is a significant threat in the realm of cryptocurrencies, where malicious actors attempt to control over half of a blockchain’s mining hash rate. This article delves into the intricacies of 51% attacks, shedding light on the potential consequences, prohibitive costs, and the safeguards that protect major cryptocurrencies like Bitcoin. We’ll explore the vulnerability of smaller networks and the implications of transitioning to proof-of-stake. By the end, you’ll have a comprehensive understanding of this critical topic.

What is a 51% attack?

A 51% attack is a security breach that occurs in the world of cryptocurrencies when a group of miners amasses more than 50% of a blockchain’s total mining hash rate. This majority control empowers them to manipulate the blockchain, potentially leading to chaos and financial losses for users.

Understanding blockchain security

At its core, blockchain is a decentralized ledger that records all transactions and secures them using cryptographic encryption. Transactions undergo a consensus validation process and are stored in blocks, each connected to the previous one through cryptographic links. This design makes it exceedingly challenging to alter transactions once they’ve been confirmed.

The mechanics of a 51% attack

A 51% attack disrupts this secure ledger by allowing the attackers, with more than half of the network’s computational power, to introduce a manipulated blockchain. This blockchain may be accepted by the network, as the attackers now possess the majority.

However, it’s crucial to understand that changing historical blocks—transactions recorded before the attack—is exceptionally difficult. The further back in time a transaction is, the more secure it becomes. Transactions prior to a checkpoint, where they become permanent, remain immutable.

Why 51% attacks are prohibitively expensive

Successfully executing a 51% attack is an arduous task, especially on widely adopted cryptocurrency networks. Attackers must control the required 51% and develop an alternative blockchain that can be inserted at precisely the right moment. Furthermore, they must outperform the legitimate network in terms of computational power.

Consider, for instance, the Bitmain S19 XP Hydro, the most advanced application-specific integrated circuit (ASIC) miner. It costs over $8,400 and boasts a hash rate of 257 terahashes per second (TH/s).

As of June 7, 2023, the top three mining pools collectively controlled 66.87% of the Bitcoin network’s hashrate, equivalent to 242.42 exahashes per second (EH/s). To match this hashrate, attackers would need over 941,634 S19 XP Hydros, amounting to nearly $7.9 billion, along with infrastructure costs.

Major cryptocurrencies, like Bitcoin, are secure against 51% attacks due to the exorbitant cost of acquiring such hashing power. Smaller networks, on the other hand, remain vulnerable.

Post-proof-of-stake Ethereum

Following Ethereum’s transition to proof-of-stake, a 51% attack on its blockchain became even more costly. Attackers would need to own 51% of the staked ETH on the network, requiring ownership of more than 9.8 million ETH (over $18 billion as of June 7, 2023). Initiating an attack would lead to immediate slashing of the staked ETH, rendering the attempt financially disastrous.

Challenges in timing the attack

Timing plays a crucial role in 51% attacks. Attackers must not only control 51% of the network but also introduce their manipulated blockchain at precisely the right moment. Large, well-established networks make this nearly impossible, as they produce new blocks rapidly, making it challenging for attackers to keep up.

Smaller cryptocurrencies with lower participation and hash rates are more susceptible to introducing an altered blockchain.

Outcomes of a successful attack

If a 51% attack succeeds, attackers can disrupt or reverse transactions, enabling double-spending. Additionally, they may launch a Denial-of-Service (DoS) attack by blocking other miners’ addresses. This keeps “honest” miners from regaining control until the dishonest chain solidifies.

Who is vulnerable to 51% attacks?

The vulnerability to 51% attacks varies based on factors like mining equipment and network size. ASIC-secured networks are less susceptible than GPU-mined ones. Cloud services, like NiceHash, provide a means to rent hash power for potential attacks, primarily targeting smaller GPU-mined networks.

Bitcoin Gold, a smaller cryptocurrency, has frequently been targeted, witnessing over 40 51% attacks since June 2019. This threat extends to other smaller cryptocurrencies as well.

Is a 51% attack on Bitcoin possible?

A 51% attack on Bitcoin is technically possible but highly improbable due to the immense cost of acquiring the required hashing power. As of June 7, 2023, attackers would need control of 181 EH/s of hashing power, equivalent to over 941,634 powerful ASIC miners costing more than $7.9 billion.


Here is a list of the benefits and drawbacks to consider.

  • Major cryptocurrencies like Bitcoin are secure against 51% attacks.
  • The high cost deters potential attackers, safeguarding the network.
  • The transition to proof-of-stake enhances security for networks like Ethereum.
  • Smaller cryptocurrencies remain vulnerable due to lower participation.
  • Attackers may exploit cloud services to target GPU-mined networks.
  • Costly attempts can lead to financial ruin for attackers.

Frequently asked questions

How can cryptocurrencies protect against 51% attacks?

Cryptocurrencies can protect against 51% attacks by maintaining a large and diverse mining network, implementing strong consensus mechanisms, and utilizing security upgrades like proof-of-stake transitions. These measures make it prohibitively expensive and challenging for attackers to control over 50% of the network’s mining power.

Are all cryptocurrencies vulnerable to 51% attacks?

No, not all cryptocurrencies are equally vulnerable to 51% attacks. Major cryptocurrencies like Bitcoin have extensive mining networks and high security measures, making them highly resistant to such attacks. Smaller and less-popular cryptocurrencies are more susceptible, as they often have lower participation rates and weaker security.

What are the consequences of a successful 51% attack?

The consequences of a successful 51% attack can be severe. Attackers can disrupt transactions, reverse them, and potentially engage in double-spending. Additionally, they may launch Denial-of-Service (DoS) attacks, causing further disruption to the network. Such attacks can erode trust in the cryptocurrency and lead to financial losses for users.

Is a 51% attack on Bitcoin realistic?

A 51% attack on Bitcoin is technically possible but highly unlikely due to the immense cost of acquiring the necessary hashing power. The Bitcoin network’s robust security and widespread adoption make it a formidable target for attackers, deterring them from attempting such an attack.

How can cryptocurrency enthusiasts protect their investments from 51% attacks?

Cryptocurrency enthusiasts can protect their investments by choosing to invest in well-established and widely adopted cryptocurrencies like Bitcoin and Ethereum. Additionally, staying informed about security measures and network upgrades within the cryptocurrency ecosystem can help individuals make informed investment decisions.

Key takeaways

  • A 51% attack is a grave threat to cryptocurrency networks, enabled by controlling over 50% of hashing power.
  • Bitcoin and Ethereum are unlikely targets due to their high security and cost barriers.
  • Smaller cryptocurrencies are more susceptible to 51% attacks, often resulting in multiple successful attempts.
  • Proof-of-stake transition enhances security against 51% attacks in some networks.
View article sources
  1. 51% Attacks – Massachusetts Institute of Technology
  2. Legal Liability of a 51%, Goldfinger Cryptocurrency Attack – Duke University
  3. An Examination of the Feasibility of a State Attack on Bitcoin – Princeton University
  4. Proteus: A DLT-Agnostic Emulation and Analysis Framework – Office of Scientific and Technical Information
  5. Exploring the Depths of Bitcoin Double Spending – SuperMoney