What is a 51% attack?
- Author
- CRYPTO PORT Editorial
- Published
- Updated
- Reading time
- 6 min
In short
A 51% attack is one party controlling a majority of the resource that secures a chain and using it to rewrite recent history in their favour. On large, well-distributed chains the cost makes it impractical; on small ones it has genuinely happened. For users, the defence is waiting for enough confirmations.
Key points
- One party holds a majority of the securing resource
- Only recent history can be rewritten, not the whole chain
- The better distributed the chain, the less affordable it becomes
- Wait for enough confirmations before treating a deposit as final
Definition
A situation in which one party controls a majority of the resource that decides who produces blocks — hash power under proof of work, staked value under proof of stake — and can therefore replace recent blocks and alter transaction history.
A blockchain reaches agreement because many participants follow the same rules and treat the history with the most resource behind it as canonical. If one party holds the majority of that resource, its version of recent history can always win, which opens the door to undoing a transaction it made.
What that power cannot do matters just as much. It cannot move anyone else's assets, because spending requires a private key and keys have nothing to do with how much hash power or stake you hold. Rewriting older blocks means redoing all the work stacked on top, so the further back a transaction sits, the more settled it is.
The chains where this is a live concern are small ones, where the securing resource is cheap to assemble. That is why exchanges require different confirmation counts for different chains. The same logic applies to you: the larger the amount and the smaller the network, the more confirmations are worth waiting for.
Proof-of-stake designs generally destroy the stake used to attack, so succeeding costs the attacker the resource itself. Under either model, the real foundation of safety is how widely the securing resource is distributed and how many independent participants there are.
Watch out for
- · On small chains, wait for extra confirmations before treating an incoming payment as settled
- · For large amounts, use the confirmation count your exchange publishes as the benchmark
- · Ignore claims that a majority attacker can take coins without keys — that is not how it works