Business Logic Flaws and Broken Access Control

Scanners cheer for missing headers. They shrug at 'coupon plus refund plus free shipping' chains that finance actually cares about.

Business logic bugs need you to understand how money, roles, and state machines are supposed to behave—then ask what happens if step three runs twice.

Logic bugs reward domain curiosity—talk to customer support flows and refund policies before assuming the dev docs tell the whole story.

Where logic breaks

Checkout flows, referral credits, trial extensions, role upgrades, and approval workflows with email links. Multi-step processes with async workers hide race windows.

Access control failures often look like logic bugs: a user performs an action the UI hid but the API still exposes.

How I explore safely

Map states on paper: cart, paid, shipped. Test transitions out of order with two browsers and two roles—not brute force.

• Read terms and help docs—they describe intended rules

• Compare mobile vs web paths

• Note admin overrides and support tools

Writing impact

Quantify fraud or data exposure, not just 'unexpected behavior.' Recommend server-side state validation and idempotent operations.

Logic findings separate testers who read from testers who scan.

Stay inside program or engagement rules—financial abuse testing may need explicit approval.

Support and admin tooling

Internal support panels impersonating users are logic bugs waiting to happen—test with the lowest support tier in scope.

Email-based approvals often skip re-auth; document whether the link alone moves money or changes roles.

Documentation as spec

Help docs and marketing copy define intended rules—quote them when behavior diverges.

Quantify fraud scenarios with round numbers clients understand; precision false confidence hurts trust.

Retest after 'quick fixes' on logic bugs—they often patch symptoms not state machines.

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Worth reading next

Race Conditions in Web Apps: Limit Checks and TOCTOU Bugs

Web Hacking for Pentesters: IDOR and Broken Object Access

Penetration Testing: Web Application Checklist (WAPT)