How Do Zero-Confirmation Transactions Increase the Risk of a Double-Spend?

Zero-confirmation transactions are transactions that are broadcast to the network but accepted by a vendor or service before they are included in a block. This increases double-spend risk because the transaction is not yet secured by the blockchain's cryptographic proof-of-work.

An attacker can broadcast a payment transaction and immediately follow it up with a conflicting transaction that sends the same funds back to themselves, which a malicious miner can prioritize.