Zero-confirmation Bitcoin payments are no longer safe for merchants

What zero-conf meant

A Bitcoin transaction is broadcast first and included in a block later, on average about ten minutes later and sometimes much longer. Waiting for that first confirmation is a poor checkout experience, so many merchants delivered as soon as the transaction was seen. The bet was that replacing an unconfirmed transaction with a different one, a double-spend, was hard: most nodes would keep the first version they saw.

What changed

Replace-by-fee (RBF) lets a sender rebroadcast a transaction with a higher fee. Originally a transaction had to opt in by signalling it, and a merchant could refuse those. With full-RBF, nodes accept a higher-fee replacement whether or not the original signalled. Bitcoin Core made full-RBF the default in version 28 (2024), and most of the network and mining pools now behave that way.

The attack is now simple: pay at a very low fee rate, wait for the shop to deliver, then broadcast a replacement that pays the same coins back to yourself at a slightly higher fee. Miners take the replacement because it pays them more. The cost is the fee difference, typically cents. A license key or download link, once delivered, is gone.

Is it still OK for small amounts?

It used to be argued that nobody bothers for a $10 item. That only holds until a script exists, and the script is a few lines. The economics no longer depend on the amount: the attack costs the same for $10 as for $1,000. For digital goods with no way to claw them back, the safe default is one confirmation.

Making one confirmation feel fast

• Show that you've seen the payment. As soon as the transaction appears unconfirmed, switch the checkout to “payment seen, waiting for 1 confirmation”. Buyers mind waiting much less once they know the money arrived.
• Don't let the order expire while it confirms. If your orders expire after an hour, extend the expiry the moment you see the payment; a low-fee transaction can take hours.
• Offer Litecoin. Blocks every 2.5 minutes means one confirmation usually arrives before the buyer has closed the wallet app, and fees are a fraction of a cent. Our fee comparer shows the difference live.
• Offer USDT on TRON. TRON confirms in about a minute; the trade-off is a fixed ~$2 fee for the sender (see the USDT guide).
• Use Lightning if you can run it. It settles instantly and safely, but needs a node and channel liquidity, which is a different operational commitment.

What we do

Crypto Checkout used to deliver Bitcoin and Litecoin payments on first sight. We changed that after a security review: orders now need one confirmation, the checkout shows the seen state, and a seen order stays open for 24 hours. It costs buyers a few minutes and removes a free double-spend.