What Is a 51% Attack on a Blockchain?

What Is a 51% Attack?

A 51% attack refers to a situation where a group of miners on a blockchain network gains control over more than 50% of the network's mining hash rate. If the attackers manage to achieve this, they can manipulate the network in various ways.

For example, they can prevent new transactions from being confirmed, effectively halting payments between users. They can also reverse transactions that were completed while they had control, potentially leading to double spending of coins and various other problems.

---

Key Takeaways

• A 51% attack occurs when one party controls more than half of a blockchain’s computing power and can manipulate the network.
• This enables them to reverse transactions, block payments, and double spend coins.
• Large networks like Bitcoin are well-protected due to their size and cost barriers, but smaller blockchains face greater risk.
• Protection methods include decentralization, Proof of Stake, network monitoring, and requiring extra block confirmations for transactions.

---

What Is a 51% Attack in Crypto?

A 51% attack (or majority attack) is a vulnerability that can occur in blockchains using Proof of Work (such as Bitcoin or Ethereum Classic). In such an attack, a single party or group gains control of more than 50% of the network's computing power. This allows them to theoretically manipulate the network as they see fit.

Let’s explore exactly what this means, why it’s dangerous, and how blockchains try to defend against it.

How Does a Blockchain Normally Work?

Before explaining the attack, it's helpful to understand how a blockchain typically functions:

• Transactions are grouped into blocks.
• Miners (computers) compete to solve a mathematical puzzle.
• The first to solve the puzzle gets to add the next block to the chain.
• The longest (most computational work) chain is considered the truth.

This structure prevents fraud.

What Happens During a 51% Attack?

If a single party gains control of more than 50% of the total computing power, they can:

• Reverse transactions they made (called double spending).
• Force a new version of the blockchain with alterations.
• Temporarily block or delay other transactions (such as those from a specific user or exchange).

They cannot:

• Steal other people's crypto.
• Create new coins.
• Modify smart contracts.

So, it’s not full control, but it does make the network unreliable.

Double Spending

Imagine an attacker sends 10,000 coins to an exchange and converts them to euros. While this happens, they secretly work on an alternative version (the compromised one) of the blockchain in which the transaction never occurred. Once they’ve withdrawn their euros, they publish the alternative chain. The network accepts this as the new "real" chain.

Result: the original transaction disappears, the coins were never truly spent, but the attacker already has the euros.

The exchange has been defrauded: the coins were double spent.

Well-Known Examples of 51% Attacks

• Ethereum Classic (ETC) has been targeted multiple times. In 2020, more than $5 million in double spends occurred.
• Bitcoin Gold and Vertcoin were also attacked.
• Bitcoin itself has never been attacked due to the immense computing power required—manipulating the Bitcoin network would cost billions of euros. This is why no one has succeeded so far.

How Can a 51% Attack Be Prevented?

1. Distributed Mining (Decentralization)
   The more independent miners there are, the harder it is to control 50% of the power.

2. Proof of Stake
   Blockchains like Ethereum have switched to Proof of Stake. The 51% attack mostly applies to Proof-of-Work networks.

3. Monitoring
   Some networks use monitoring tools to quickly detect unusual behavior.

4. Confirmations
   Exchanges often wait for multiple block confirmations before finalizing a transaction, limiting the damage of a potential double spend.

Conclusion: How Big Is the Risk?

For large blockchains like Bitcoin, a 51% attack is theoretically possible but practically unfeasible due to the massive costs and energy required. For smaller coins, the risk is higher: fewer miners = more vulnerability.

A 51% attack is a serious security threat, especially for low-activity blockchains. That’s why decentralization is crucial in designing any trustworthy blockchain.