Business Logic Flaws in Authentication Flows

Business logic flaws are vulnerabilities in how an application's rules and workflows fit together, rather than in any single line of code. In authentication flows they are especially dangerous because the logic guards the front door. A password reset, a multi-step verification, or a rate limit can each be individually correct yet combine into a sequence an attacker can abuse to take over an account.

Analogy🏏Cricket

🍳 Think of it like cooking: A recipe can look flawless on paper, but a head chef only truly judges the dish by tasting the finished plate, because heat, timing, and the actual oven reveal problems the written method never shows. Just as tasting the cooked result catches what reading the recipe cannot, DAST probes the running application and catches the misconfigurations and runtime behaviour that reading the source cannot. This reveals DAST's stance: it judges the dish as it is actually served, not the recipe as it was merely written.

What makes these flaws slippery is that scanners rarely find them: there is no injectable character or missing header, only a workflow that assumes users behave as intended. Attackers do not behave as intended. This lesson looks at common auth-logic failures, how skipping or reordering steps breaks them, and why defending them requires reasoning about abuse cases, not just testing individual requests.

Analogy🏏Cricket

💰 Think of it like finance: A real audit that actually attempts a sample transaction proves beyond doubt whether a control can be bypassed, with little room for a false alarm, but it can only test the accounts and processes the auditors actually reach. Just as a hands-on audit yields high-confidence, low-false-positive results yet is bounded by what it examines, DAST yields high-confidence findings about real exploitability yet is bounded by the surface it can reach. This reveals its trade-off: what it confirms is solid, but it only confirms what it manages to touch.