Business Logic Flaws: How Attackers Skip Steps in Your App to Get What They Should Never Have Business logic flaws are vulnerabilities that arise when an application trusts its own workflow too much, allowing attackers to skip, repeat, or reorder steps to achieve unintended outcomes without exploiting traditional coding errors. A common example is payment bypass, where an attacker directly requests a confirmation endpoint that grants Pro status without verifying that a payment transaction was completed. To prevent such flaws, developers must validate every critical action on the server side, such as checking for a completed transaction before updating user privileges. Business logic flaws are vulnerabilities that exist not because of a coding mistake, but because the application trusts its own workflow too much. Instead of exploiting a buffer overflow or injecting code, attackers simply skip steps, repeat actions, or change the order of requests to get outcomes the app never intended. This kind of flaw is common in multi-step processes like account upgrades, order flows, approval chains, and access control checks. Payment bypass is one well-known example, but the same root cause appears across many different features. Logic flaws happen when applications trust the order of steps instead of verifying each action on the server side. Attackers can skip, repeat, or reorder requests to reach states the application assumed were impossible to reach. Common scenarios where this shows up: The example below uses a payment bypass to show how this works. The application has a three-step upgrade process: select a plan, submit payment, then confirm activation. The confirmation endpoint does not verify that payment was actually completed before granting Pro status. The diagram shows how an attacker can skip step 2 by requesting the confirmation endpoint directly. The vulnerable code assumes that if a user reaches the confirmation page, they must have paid. It grants Pro status without checking for a valid transaction. // /upgrade/confirmed.php // Logic flaw: The code assumes if you are here, you must have paid. $user id = $ SESSION 'user id' ; // Directly updating the database to 'pro' status $sql = "UPDATE users SET membership = 'pro' WHERE id = '$user id'"; $db- query $sql ; echo "Congratulations You are now a Pro member."; An attacker can bypass the payment step by sending a direct request to the confirmation endpoint: curl http://