A 51% attack is an attack on a blockchain in which one party, or a coordinated group, controls more than half of the network's total HashrateHashrateThe total number of guesses a network's mining computers make each second, and a measure of how well the network is protected from attack.. The attacker can then find blocks faster than everyone else combined. That makes it possible to build their own version of the chain, which eventually replaces the one everyone else built. On networks that use proof of stake, the attacker would instead need to control a majority of the crypto staked.
What can an attacker do?
- Double-spend their own coins. For example, they can deposit crypto at an exchange, sell it and withdraw the money. Then they publish their own chain, in which the deposit never happened. This is double spendingDouble spendingSpending the same digital money twice; the problem Bitcoin was the first system to solve without a bank or other middleman..
- Block transactions. They can stop particular transactions from getting into blocks.
- Shut others out. They can ignore blocks from other miners and take all the rewards themselves.
What can't they do?
The Bitcoin WhitepaperWhitepaperA document in which the creators of a cryptocurrency or project describe how it works, what it is for and which rules apply to it. points out that even a successful attack has its limits. The attacker can't:
- take crypto that was never theirs, because they don't have other people's private keys
- create coins out of thin air or change the network's rules, because nodes reject invalid blocks
- change old transactions without enormous effort, because each new block on top of a transaction adds a ConfirmationConfirmationA sign that a transaction has been included in a block; each new block added on top counts as one more confirmation and makes it more secure. and increases the work that would have to be redone
Examples
Nobody has ever pulled off a 51% attack on Bitcoin. It would take more hardware and electricity than all the world's other miners have combined.
Smaller networks that use proof of workProof of workA consensus method where the right to add the next block to a blockchain goes to whoever first solves a hard computing puzzle. are more exposed:
- Bitcoin Gold suffered 51% attacks in May 2018. They targeted crypto exchanges, with the aim of double-spending deposits.
- Ethereum Classic suffered a 51% attack in January 2019 and three more in the summer of 2020. Millions of dollars were double-spent.
In response, many exchanges required far more confirmations for these coins. After the 2020 attacks, some made people wait up to two weeks for Ethereum Classic deposits.
Everything in Myntkaup Learn is written and regularly reviewed by the Myntkaup editorial team. It is for education and is not investment advice. Editorial policy. Updated 18 October 2026.