In one sentence
A 51% attack is an attempt by someone who controls the majority of a blockchain's block-producing power — computing power in proof of work, or staked deposits in proof of stake — to rewrite recent history or block certain transactions.
How it works
In proof of work, nodes follow the valid chain with the most accumulated work. An attacker with more computing power than the rest of the network combined can quietly build a private branch faster than everyone else and then publish it. Nodes switch to the heavier branch, and the blocks it replaces — with their transactions — disappear from the main history.
The classic use is a double spend: pay someone, wait until they deliver, then publish a branch in which that payment never happened and the same units went elsewhere. The attacker can also refuse to include certain transactions.
What a majority cannot do matters just as much. It cannot forge signatures, so it cannot move other people's units. It cannot create units beyond the rules or include invalid transactions, because every full node would reject such blocks.
The cost of an attack grows with the size of the network. Small proof-of-work networks, especially those whose mining hardware can be rented, are far more exposed; Ethereum Classic suffered such reorganisations in January 2019 and again in August 2020.
In proof of stake, the thresholds differ: a third of staked deposits can stall finality, and rewriting finalised history would require at least a third of all deposits to be destroyed by slashing.
A simple example
An attacker sends units to a merchant, who delivers after two confirmations. Meanwhile the attacker's machines have built a private branch, three blocks long, in which those units were sent back to the attacker. Once it is published, nodes adopt the longer branch and the merchant's payment vanishes from the record.
Common misconception
Misconception: With 51% of the power, an attacker can take anyone's funds.
In fact: Majority control lets an attacker reorder or remove recent blocks, mainly to reverse their own payments. Moving other people's units would still require their private keys.
Where you meet it
- Advice on how many confirmations to wait for.
- Security analyses of smaller networks.
- Section 11 of the Bitcoin whitepaper, which calculates the odds.
Related cards
Sources
This card explains a technical concept for general education. It is not a recommendation to acquire, sell or hold any crypto-asset, and it is not financial advice.