Race Attack

Intermediate
Жаңыртылган Jul 9, 2026

What Is a Race Attack?

A race attack is a type of double-spend in which an attacker creates two conflicting transactions that spend the same funds at the same time, then broadcasts them in a way designed to deceive a recipient. 

The goal is to make a merchant believe they have been paid, while a second, conflicting transaction sends the same coins back to the attacker or to another address.

Race attacks specifically target recipients who accept an unconfirmed transaction as final, often called a zero-confirmation payment. 

Because the network has not yet permanently recorded which transaction is valid, there is a brief window where the outcome can be influenced. This makes the attack most relevant in fast checkout situations where a seller releases goods or services immediately.

How Does a Race Attack Work?

In a typical scenario, the attacker sends one transaction to a merchant and, at nearly the same moment, broadcasts a competing transaction that spends the same coins to themselves. Both transactions enter the memory pool (mempool) of different nodes across the network. 

Since nodes may receive the two transactions in different orders, they temporarily disagree about which one is legitimate.

Only one transaction can ultimately be included in a block. If miners confirm the transaction that returns the coins to the attacker, the payment to the merchant becomes invalid. 

The attacker may improve their chances by connecting directly to the merchant’s node to deliver the paying transaction, while broadcasting the conflicting transaction more widely to the rest of the network. If the merchant accepted the payment before any confirmation, they could be left with nothing.

How to Reduce the Risk of a Race Attack

The most effective way to reduce exposure to a race attack is to wait for one or more block confirmations before treating a payment as final. Once a transaction is included in a block and additional blocks are built on top of it, reversing it becomes increasingly difficult and costly.
Merchants handling larger amounts often wait for several confirmations to lower the probability of reversal. It is worth noting that a race attack is different from a 51% attack, which requires control of a majority of a network’s mining power and can rewrite already confirmed transactions. A race attack does not require that level of resources, but it generally only succeeds against zero-confirmation acceptance.

In practice, waiting for confirmations, monitoring for conflicting transactions, and avoiding instant acceptance of unconfirmed payments can substantially reduce the likelihood of a successful race attack.

Learn more: Double Spending Explained
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