What is a majority attack on a blockchain?
A majority attack is when one miner or pool controls most hash rate and rewrites blocks. It can double-spend but cannot steal coins from wallets.
Vahe HakobyanEditor-in-chief
Updated Oct 6, 20263 min readFact-checked
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People also call it a majority attack. The name comes from the share of mining power needed, which is more than half. Attackers usually rent hash power or pool miners together instead of owning most of the coins.
How does a majority attack work?
The attacker rents hash power or pools miners to control most of the chain's hash rate. They mine a private chain in secret. If it grows longer than the public chain, they broadcast it. Other nodes follow the longest chain, so the private chain replaces the public one.
What can attackers do with it?
Once the attacker's chain becomes the main chain, they can change recent history. The main actions are:
- Double-spend their own coins by paying, then releasing a chain where the payment never happened.
- Reverse their own recent transactions by replacing blocks that confirmed them.
- Block other people's transactions by leaving them out of blocks they mine.
What are its limits?
A majority attacker cannot steal coins from your wallet because they do not have your private keys. They cannot create new coins or change the rules on their own. Small proof-of-work chains are easier targets. Bitcoin has run since early 2009, and attacking its large network is very costly.
How is it different from a double-spend?
A majority attack is the method. A double-spend is one possible outcome. The attacker uses majority hash power to rewrite the chain and spend the same coins twice. A double-spend can also happen without a majority attack. In March 2013, a bug in the Bitcoin 0.8.0 client caused the chain to split, and Bitcoin saw the first known double-spend.
Frequently asked questions
It is possible in theory, but no majority attack on Bitcoin has been observed. Bitcoin's proof-of-work protocol has probabilistic finality, meaning transactions are never technically final because a conflicting chain can always outgrow the current chain.
Your coins are not stolen because an attacker does not have your private keys. A payment you already made can be reversed, so the merchant who accepted it loses the funds.
No. A Sybil attack uses many fake identities to influence a network. A majority attack uses control of most hash power to rewrite blocks.
In proof of stake, an attacker would need most of the staked coins instead of hash rate. The result can be similar: rewriting blocks or double-spending.
