Double Spend

A double spend is an attempt to use the same coins in two conflicting transactions. Public blockchains prevent this by agreeing on one transaction history and rejecting later attempts to reuse already-spent inputs.

The main risk window is before a transaction is included in a block. Mempool policies, replacement rules, propagation speed, and merchant monitoring all affect how safe zero-confirmation acceptance is.

Once a transaction is confirmed, reversing it usually requires a chain reorganization. Each additional block built on top increases the cost and difficulty of replacing the confirmed transaction.

A majority-hash-rate attacker can attempt to mine a private chain that excludes a payment, then publish it after receiving goods or exchange credit. This is expensive on large proof-of-work networks but more realistic on smaller chains with rentable hash power.