The application doing exactly what it was built to do, for the wrong person.
What gets tested: Workflow and state abuse. Steps executed out of order, skipped approvals, replayed transactions, flows entered from the middle, sequences that were never meant to chain. Scraping, enumeration, and value extraction through legitimate calls.
Why it's hard: Every request is valid. Nothing is malformed. There is no signature to match, because the attack is a sequence, not a payload. This is the layer that generic tooling cannot reach, and it requires knowing how the application is supposed to behave before you can prove it behaved wrongly.