What is a 51% Attack on the Blockchain?

A 51% attack is a type of attack against blockchain that occurs when a miner (a single attacker) or group of miners gain control of more than 50% of the network’s total mining/hashing power, allowing them to manipulate the blockchain and double-spend coins. This type of attack is particularly dangerous because it allows the attacker to reverse transactions and prevent new transactions from being confirmed.

Ethereum Classic blockchain

One of the major incidents of a 51% attack occurred in January 2019 on the Ethereum Classic blockchain. An unknown attacker was able to control more than 50% of the network’s mining power and used it to double-spend coins, reversing transactions and stealing more than $1 million worth of cryptocurrency.

Additionally, in August 2021, the Ethereum Classic blockchain was hit by another 51% attack incident where the attacker used their control over the network to double spend $5.6 million worth of cryptocurrency.

Bitcoin Gold blockchain

Bitcoin Gold, a cryptocurrency that forked from the original Bitcoin blockchain in 2017, experienced a 51% attack. This resulted in the double-spending of over $72,000 worth of Bitcoin Gold tokens. This was the second time that Bitcoin Gold has suffered such an attack, with $18 million worth of tokens being stolen in May 2018.

Bitcoin SV blockchain

In the case of Bitcoin SV (BSV), there have been several instances of 51% attacks reported in August 2021.

by Lucas Nuzzi, a network data product manager at Coin Metrics

But according to the Bitcoin Association, the hacker’s attempts to perform a 51% attack were not successful.

Other major incidents

In addition to these incidents, there have been several other instances of 51% attacks on other smaller blockchain networks such as ZenCash, Monacoin, and Verge. For example, in April 2018, the ZenCash blockchain fell victim to a 51% attack, resulting in the attacker stealing more than $500,000 worth of cryptocurrency. Similarly, in May 2018, the Monacoin blockchain was targeted in a 51% attack, resulting in the attacker stealing more than $90,000 worth of cryptocurrency.

How to fight 51% attacks

These attacks also highlights the importance of implementing security measures such as checkpointing, which allows the network to periodically take a snapshot of the blockchain and use it as a reference point to detect and prevent any malicious activity.

Another way of mitigating such type of attack is to use a different consensus mechanism which is more resistant to 51% attack like Proof of Stake (PoS) which do not rely on hashing power to secure the network.


The 51% attack is a serious threat to the security of blockchain networks, particularly smaller networks. It is crucial for blockchain networks to take steps to prevent such attacks and for users to be aware of the risks associated with small and under-secured blockchain networks.

M. Ali

Daily cryptocurrency trader, miner, technology enthusiast and a full time IT and security consultant. If you have any questions or comments please feel free to email him at

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