Why Are Zero-Confirmation Transactions Considered Less Secure than Confirmed Transactions?
Zero-confirmation transactions are those that have been broadcast to the network but not yet included in a block. They are vulnerable to double-spending attempts, especially via RBF, because they are not yet secured by the network's consensus mechanism.
A malicious user can replace the unconfirmed transaction with a conflicting one before a miner includes the original.
Glossar
Zero-Confirmation
Risk ⎊ Accepting a transaction before it is included in a confirmed block exposes the recipient to the immediate risk of transaction reversal via a chain reorganization.
Transactions
Recording ⎊ Transactions are the atomic units of value transfer or state change recorded immutably on the distributed ledger, forming the basis of all financial activity including derivatives settlement.