Business logic bugs are where scanners fail and pentesters earn their fee. They’re not about malformed input — the input is well-formed. They’re about intended features being used in combinations or sequences the developers didn’t model. This module teaches you to see the game rules of an application so you can find moves the designers didn’t anticipate.
Why this happens
Developers model the “happy path.” They write the test suite for the happy path. Reviewers approve the happy path. QA validates the happy path. Nobody models the entire state space, because the state space is exponential in the number of features. Business logic bugs live in state combinations — a sequence of legal actions that together produce an illegal outcome.
Classic example: coupon + refund. Each feature is fine in isolation. Together: use coupon, get discount, refund original item, keep the discounted second item. Each step is an allowed action. The combination nets free merchandise. No scanner finds this. No test case covers it until someone thinks to write it.
How these bugs emerge
Three archetypes:
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