Networks settle disagreements by following whichever valid chain has the most accumulated work or the heaviest set of attestations. If one party commands a majority of that power, it can build a competing chain privately and then publish it, and honest nodes will switch to it because the rules say to. Recent blocks are replaced — a reorganisation — and any transaction that existed only in the discarded blocks disappears.
The classic payoff is a double spend. The attacker deposits coins somewhere, waits for the withdrawal or the goods, then releases a chain in which the deposit never happened, ending up with both. They can also censor, refusing to include particular transactions for as long as they hold the majority.
Now the limits, because this is widely misunderstood. Majority power confers no ability to forge signatures, so an attacker cannot move coins from an address whose private key they do not hold. They cannot mint coins outside the issuance rules or lift a supply cap: every node validates each block independently and rejects invalid ones, whatever power sits behind them. And they cannot casually rewrite deep history — the further back a transaction sits, the more work redoing it demands.
The name is a rounding, not a switch. Below a majority an attacker succeeds only some of the time; above it, reliably. Cost is what really protects a chain, which is why networks with modest <a href="/glossary/hash-rate/">hash rate</a> are the ones that have been hit — power can be rented, so security tracks the price of renting it rather than the technology alone. See <a href="/glossary/proof-of-work/">proof of work</a> for how that budget is built and <a href="/glossary/decentralization/">decentralisation</a> for why spreading production out matters. Waiting for more confirmations on large payments remains the standard defence.
Key takeaways
- A majority attacker can reorder and censor recent transactions but cannot sign transactions for keys they do not hold.
- Invalid blocks are rejected by every node no matter how much power backs them, so supply and issuance rules cannot be broken this way.
- Deeper confirmations are the practical defence, and they matter most on smaller chains where majority power is cheap to rent.
51% Attack — frequently asked questions
Could someone with a majority steal my coins?
Not directly. Spending from your address requires your private key, and no amount of block-production power substitutes for a valid signature. What a majority attacker can do is undo recent transactions involving their own coins, keep yours out of blocks, and profit at the expense of whoever accepted a payment that later vanished. Exchanges and merchants carry more of that risk than ordinary holders.
Why do exchanges require more confirmations for some coins than others?
Because the cost of rewriting recent blocks differs enormously between networks. On a chain that commands a small share of the power available for hire, a handful of blocks can be redone relatively cheaply, so the exchange waits longer before treating a deposit as final. Requiring more confirmations is simply buying more of the work an attacker would have to repeat.
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