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What Is a 51% Attack? What It Can and Cannot Do
A 51% attack is when one miner or group controls a majority of a proof-of-work chain's hashing power and uses it to rewrite recent blocks, for example to reverse its own payments.
Key takeaways
- With a majority of hashrate, an attacker can out-build the honest chain and replace recent blocks.
- The main use is double-spending the attacker's own transactions after the fact.
- It cannot create coins out of nowhere, change the rules nodes enforce or spend coins without the private keys.
- Cost scales with hashing power, so small chains are cheaper to attack than large ones.
What the attacker controls
Proof-of-work chains pick the valid chain by accumulated work. If one party controls more than half of the hashing power, it can privately mine a longer branch than the honest network and then publish it. Nodes follow the branch with more work, so the honest blocks it replaces are dropped. The original Bitcoin paper analyses this race and shows the attacker's chance of catching up falls sharply as the honest chain gets deeper, provided the attacker has less than half the power. With a majority, the race is eventually won.
What an attacker can do
The practical harm is a double spend. The attacker pays for something, waits for it to be delivered, then publishes a longer branch in which that payment never happened and the coins are spent elsewhere. They can also block specific transactions from confirming while they hold the majority.
What an attacker cannot do
A majority of hashrate does not let anyone spend coins they lack keys for, because signatures are checked by every node. It does not let them mint extra coins or change the issuance schedule, because nodes reject blocks that break the rules. The damage is to the finality of recent transactions, not to ownership of old coins.
Why exchanges and merchants use more confirmations on small chains
The cost of an attack is the cost of renting or owning enough hashing power. Large chains make that enormously expensive; small chains can be far cheaper. That is why services typically ask for more confirmations on lower-hashrate coins, and why merge-mined chains, which share hashing power with a larger parent, are treated differently.
Example
A merchant accepts 1,000 coins on a small chain after two confirmations. The buyer controls a majority of that chain's hashrate, secretly mines a longer branch where the coins went back to themselves, then publishes it. The merchant's payment disappears, and the goods are already gone.
Put it into practice
Watch confirmations on every incoming payment
For payments you cannot reverse, confirmations are your protection. Scrypt Wallet lets you hold coins with keys only you control, and you can check each payment on a block explorer before you rely on it.
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Last reviewed 2026-10-02 by the Scrypt Wallet team. The factual claims on this page, and where each comes from:
- An attacker with a majority of hashing power can out-build the honest chain. [1] [2]
- The attacker's chance of catching up from behind falls as the honest chain gets longer, if the attacker has less than half the power. [1]
- The attack targets recent transactions (double spending), not ownership of coins without keys. [1] [2]
Sources
- Bitcoin: A Peer-to-Peer Electronic Cash System · Satoshi Nakamoto, 2008 · primary source
- Weaknesses · Bitcoin Wiki · secondary source
Primary sources are protocol papers, specifications, source code or a project's own documentation; secondary sources are wikis and explainers. We checked each claim against the source text on the review date. Found an error? Tell us and we will correct it.
Frequently asked questions
Can a 51% attack steal my coins from my wallet?
Not directly. Spending your coins needs your private key signature. An attacker can reverse recent transactions on the chain, which mainly puts payments you accept at risk.
Does exactly 51% matter?
The name is shorthand for a majority. An attacker with less than half still has some chance of reversing a transaction with few confirmations, and the chance drops quickly with depth.
How do I reduce the risk when accepting payments?
Wait for more confirmations, scaled to the amount and to the chain's total hashing power.
Written by the Scrypt Wallet team · Updated 2026-10-02. General education, not financial, tax or legal advice.