Topic · How blockchains work
What is a 51% attack?
- Quick fact 1
The bitcoin whitepaper assumes honest nodes control most CPU power for the system to be secure.
Source: Bitcoin whitepaper (bitcoin.org)
- Quick fact 2
ethereum.org explains that attackers with large shares of staked ether could try to reorganize or delay finality, at the risk of heavy penalties.
- Quick fact 3
Proof of work makes it expensive to rewrite blocks because the work has to be redone.
The short lesson
A blockchain is secure when the honest participants outweigh any attacker. A 51% attack is what can happen when that stops being true.
On a proof-of-work network, a group with more than half the total hash rate could secretly build a longer chain and then publish it, replacing recent blocks. That lets them double-spend: pay an exchange, withdraw, then erase the payment.
What an attacker cannot do
- Steal coins from your address without your private key.
- Create coins out of thin air beyond the rules.
- Change very old history cheaply.
Proof of stake
On Ethereum, an attacker would need a large share of staked ether, and the protocol can destroy (slash) stake that breaks the rules. It is expensive by design.
Why it matters to beginners
Smaller networks with little mining power or stake are easier to attack. This is one reason exchanges ask for more confirmations on some coins.
Lesson game · earn XP
Check yourself: 3 quick questions
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Memory · spaced repetition
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Related terms
Go deeper in Lesson 4: The shared list everyone can check
More in How blockchains work
Common questions
Has bitcoin had a 51% attack?
Bitcoin’s large hash rate makes one extremely expensive. Some smaller proof-of-work coins have suffered them.
Can a 51% attack steal my coins?
Not directly from your address. The main risk is reversed recent payments.
Is proof of stake immune?
No system is immune. Proof of stake uses penalties and finality to make attacks very costly.
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Written and reviewed by Joshua Baskin, founder. Last reviewed . How we check facts · Report an error
Education only. Not investment, tax or legal advice. We do not pick coins.