A 51% attack is a potential security threat to blockchain networks, particularly those using Proof-of-Work (PoW) consensus mechanisms. It occurs when a single entity or group gains control over more than 50% of the network’s mining hash power, allowing them to manipulate transactions and disrupt the blockchain’s integrity.
How a 51% Attack Works
Majority Control – The attacker gains dominant mining power, enabling them to:
Reverse transactions (double-spending).
Exclude or modify new transactions (censorship).
Prevent other miners from confirming blocks.
Double-Spending Risk – The attacker can spend cryptocurrency, then reverse the transaction while keeping the coins, essentially spending them twice.
Blockchain Reorganization – The attacker can create an alternative chain longer than the legitimate one, forcing the network to accept their version.
Which Blockchains Are Vulnerable?
Smaller PoW-based cryptocurrencies (e.g., Bitcoin Gold, Ethereum Classic) are more at risk because they have lower hash power, making it easier and cheaper to attack. Large networks like Bitcoin are highly resistant due to their massive mining power.
Preventing 51% Attacks
Shift to Proof-of-Stake (PoS) – Ethereum’s transition reduces this risk.
Increased decentralization – More miners make attacks harder.
Checkpointing – Trusted nodes can reject malicious chains.
A 51% attack undermines trust but doesn’t destroy a blockchain permanently. Strong security measures and decentralization help prevent it.
I would like to take the opportunity to correct some common misconceptions about 51% attacks:
Getting 51% of the mining power does not guarantee you can alter the blockchain, any more than buying 51% of raffle tickets guarantees that you’ll win.
I’ve heard from mining experts that, for practical purposes, it’s more like a ‘66% attack’
Hashrate is a poor indicator, because it depends on the algorithm.
A better indicator is megawatts of power at your disposal.
Coins such as bitcoin gold (BTG) and ethereum classic (ETC) implemented other ways to thwart brute-force mining attacks. That’s why they don’t get attacked these days.
As Satoshi said, reversing the blockchain does not give you coins you never had- you wouldn’t even get into the mempool with that tactic; however, it does allow you to get goods (including dollars) in return for nothing.
The longer you wait, the more blocks append to the one with your transaction, and the harder it is for an attacker to reverse it
Hence, if you’re buying a coffee, wait for one confirmed official block, but if you’re buying a Ferrari, wait for 12 or more.
The bigger problem is actually a constant fear that this will happen, undermining trust in the system.
The real danger is the 51% attack of nodes.
As soon somebody has more than 50% of all nodes in the network the blockchain can modified and the original nodes which hold the real blockchain but are less then 50% would follow the mainstream and mark the own chain as wrong
If the attacker breaks with the blocksignature the whole chain is broken