What Is the ‘Double-Spend’ Problem That a 51% Attack Exploits?

The double-spend problem is the risk that a digital currency can be spent more than once. Unlike physical cash, digital assets can be duplicated.

Bitcoin's blockchain solves this by using a public ledger where all transactions are verified and ordered by a decentralized network of miners. A 51% attack exploits this solution by overpowering the network with a fraudulent version of the ledger.

By creating a longer, alternate chain of transactions in secret, the attacker can make their initial transaction disappear after it has already been accepted, allowing them to spend the same funds a second time.