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A zero-confirmation transaction (also called a 0-conf transaction) is a crypto transaction that has been broadcast to the network but has not yet been included in a mined block. It exists in the mempool, which is the waiting area for unconfirmed transactions, and has received zero block confirmations.
Bitcoin blocks are mined roughly every 10 minutes. Waiting for even one confirmation means waiting up to 10 minutes before a merchant knows a payment has arrived. For physical retail settings, that delay is impractical. Zero-confirmation transactions let merchants see a payment immediately and decide whether to accept it before the block confirms.
Most nodes on the Bitcoin network follow a rule called first-seen policy. Once a transaction is broadcast, the first version seen gets relayed and recorded. This makes it difficult (though not impossible) to replace it with a conflicting transaction. Many merchants accept 0-conf for low-value purchases based on this assumption.
Accepting a zero-confirmation transaction carries real risk. Because the transaction is not yet locked into the blockchain, an attacker can attempt to replace it with a conflicting version that sends the same funds to a different address, effectively spending the same coins twice. This is called a double-spend attack.
Bitcoin's Replace-by-Fee (RBF) feature makes this risk more concrete. RBF allows a sender to rebroadcast a transaction with a higher fee to incentivize miners to include the new version instead of the original. If you accept a 0-conf transaction from an RBF-enabled wallet, the sender can broadcast a replacement before the first transaction confirms and potentially redirect the funds away from you.
The practical risk of a double-spend depends heavily on transaction value and context. Researchers at Bitcoin Magazine and academic institutions have consistently found that successful double-spends against 0-conf are rare, costly to execute, and not worth the effort for small purchases.
Many payment processors, including Bitcoin Lightning Network gateways and some Bitcoin payment service providers, have built risk-scoring systems that assess 0-conf transaction safety in real time. These systems check whether the sender's wallet has RBF enabled, whether the transaction carries a competitive fee, and whether the same inputs appear in any conflicting transaction already propagating through the network.
The Lightning Network provides instant, final payments without relying on 0-conf trust. Payments routed through Lightning are settled off-chain through pre-funded payment channels and only touch the Bitcoin blockchain when channels open or close. For merchants who need sub-second payment finality with no double-spend risk, Lightning is the standard solution as of 2026.
https://bitcoin.org/en/developer-guide#detecting-forks
https://bitcoinmagazine.com/technical/replace-by-fee-explained
https://lightning.network/lightning-network-paper.pdf
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