Understanding 51% Attack on Blockchain

June 04, 2023 / 5 min read / - views​


No system is entirely immune to vulnerabilities and the same is true with Blockchain and one such concern is the 51% attack. A 51% attack is capable of wreaking havoc on a blockchain network and has the potential to undermine one of the core principles of blockchain technology: decentralization. In this blog, we will understand what exactly is a 51% attack and how it is prevented in the blockchain network.

What is a 51% Attack?

A 51% attack occurs when a single malicious actor or a group of adversaries manages to gain control of more than 50% of a blockchain network's mining power, also known as hashing power. By controlling the majority of the network's hashing power, the attackers can manipulate the consensus algorithm and commit malicious acts, such as double-spending and blocking transactions. These attacks pose a significant threat to the integrity of public blockchain networks and can lead to a loss of trust in the system.

The Role of Consensus Algorithm?

Blockchain networks rely on a consensus algorithm to validate transactions and maintain the security and integrity of the system. Consensus algorithms like Proof of Work (PoW) and Proof of Stake (PoS) require nodes to agree on the validity of transactions before adding them to the blockchain. This process ensures that the network remains decentralized and secure from tampering.

Centralization vs. Decentralization

A key feature of blockchain technology is its decentralized nature, meaning that no single entity has complete control over the network. Decentralization ensures that all participants on the network must come to an agreement on the current state of the blockchain. This process provides a level of security and trusts in the system, as no single entity has the power to manipulate or control the network.

However, when a 51% attack occurs, this decentralization is compromised, as the attacker gains control over the majority of the network's hashing power. This control allows the attacker to manipulate the consensus algorithm, leading to potential double-spending and transaction blocking.

How Does a 51% Attack Work?

In a 51% attack, the malicious actor(s) leverage their majority control over the network's hashing power to manipulate the blockchain in their favor. This control enables them to:

  • Double spend their coins.
  • Prevent transactions from being confirmed.

However, there are certain actions a 51% attacker cannot perform:

  • Reverse confirmed transactions.
  • Create false transactions (that never occurred).
  • Steal funds from a specific address.
  • Create new coins out of thin air.

Double Spending

Double spending is the act of using the same coins for multiple transactions. In a 51% attack, the attacker can send coins to one person while simultaneously sending the same coins to another person. While the transaction sent to the first person is publicly displayed on the blockchain, the attacker secretly mines blocks for the double-spent transaction to the second person without exposing it to the rest of the network.

Once the first transaction receives a few confirmations on the public blockchain, the attacker can reveal their secretly-mined blocks containing the double-spent transaction to the second person. Since the attacker controls the majority of the hashing power, they can create a longer blockchain than the original one, validating their double-spent transaction and effectively canceling the first transaction. This results in the first person receiving no funds, while the attacker gains the benefits of both transactions.

Blocking Transactions

By controlling the majority of the hashing power, a 51% attacker can decide which transactions to include in the next block. This power allows the attacker to create empty blocks or selectively include and exclude transactions, effectively blocking transactions from being confirmed.

Unlike PoW, PoS does not require extensive computational power. Instead, it relies on the concept of economic stake to secure the network. Nodes with higher stakes have a stronger incentive to act honestly, as they risk losing their stake if they engage in fraudulent behavior. This aspect makes PoS more energy-efficient compared to PoW.

The Real-World Impact of a 51% Attack

A successful 51% attack can have severe consequences for the attacked blockchain network. Users may lose their digital assets, and the public's trust in the network may erode. The price of the affected cryptocurrency could plummet as people begin to sell off their holdings in fear of further attacks.

The general assumption is that a 51% attack would effectively kill the targeted digital currency. However, the reality is often more nuanced, as some blockchain networks have managed to recover from such attacks and implement measures to prevent future occurrences.

How to Prevent a 51% Attack?

  • Decentralization of Miners

    The primary defense against a 51% attack is the decentralization of miners. As long as no single entity controls more than 50% of the mining power, the network remains secure. Robust blockchains, like Bitcoin, are considered inherently safe due to the vast amount of mining power distributed across the network.

  • Incentive Structure

    An attacker with significant mining power would likely make more money by mining legitimately than by launching a 51% attack. This economic incentive structure reduces the risk of such an attack substantially.

  • Consensus Mechanisms

    Adopting alternative consensus mechanisms, such as Proof of Stake (PoS) or Delegated Proof of Stake (DPoS), can help prevent 51% attacks. These mechanisms rely on validators with a stake in the network, rather than miners with computational power, to secure the network and validate transactions.


A 51% attack poses a significant risk to blockchain networks, particularly those without substantial diversification of hashing power. Large networks like Bitcoin and Ethereum are less prone to these attacks, but smaller networks and altcoins must remain vigilant.