Card testers try stolen cards at checkout until one is approved. Those attempts still reach your payment processor unless Shopify stops them first. Checkout Firewall turns your block list, trust list, and shared fraud matches into a checkout rule. A match can be stopped before payment. The app never sees card numbers. Checkout never calls an outside service. New installs start in Monitor and only log what would have been blocked. Switch to Balanced or Aggressive when you want checkout blocked.

• Card-testing matches are stopped in checkout, before payment is attempted.
• Monitor only logs checkouts that would be blocked. Buyers are not stopped.
• Block or trust an email, phone, or address for this store only.
• Get an email when a card-testing attack is detected on your store.
• Shared fraud matches never show another store's customer details.