Double Spend
In simple terms
A double spend is paying two people with the same money. Digital files copy perfectly, so the whole point of a blockchain is to make one agreed order of transactions that stops it.
Definition
Spending the same digital coins twice, which blockchain consensus exists to prevent.
In depth
Double spending is the fundamental problem of digital cash: without a trusted ledger keeper, nothing stops a payer broadcasting two conflicting transactions spending the same output. Consensus resolves it by establishing a single canonical order, so only the first spend is valid and the second is rejected as referencing an already-spent output. The attack surface is therefore not the confirmed ledger but the window before confirmation, and against a chain reorganisation deep enough to unwind a confirmed transaction.
How does Double Spend work?
An attacker broadcasts a payment to a merchant and, at the same time, a conflicting transaction returning the funds to themselves. Whichever a block producer includes first becomes valid and the other is discarded. A merchant accepting zero confirmations therefore takes real risk. Waiting for confirmations means an attacker would additionally have to reorganise the chain, which is what makes deeper confirmation thresholds a defence.
An example
A merchant releases goods on an unconfirmed transaction. The attacker had simultaneously broadcast a conflicting transfer to their own address at a higher fee; that one is included in the block and the merchant's is discarded. Waiting for even one confirmation would have prevented it.
Figures are illustrative only.
What beginners get wrong
- Accepting payment at zero confirmations for anything of value.
- Assuming a wallet showing "pending" means the money is safe. Pending means unsettled.
- Thinking this is a flaw in blockchains. Solving it is the reason they were invented.
Related terms
Part of
How do crypto scams work, and how do you avoid them? — the subject page for security and scams, with all 17 of its definitions in one place.
Educational only — not financial advice.