--- name: business-logic description: Business logic testing for workflow bypass, state manipulation, and domain invariant violations --- # Business Logic Flaws Business logic flaws exploit intended functionality to violate domain invariants: move money without paying, exceed limits, retain privileges, or bypass reviews. They require a model of the business, not just payloads. ## Attack Surface - Financial logic: pricing, discounts, payments, refunds, credits, chargebacks - Account lifecycle: signup, upgrade/downgrade, trial, suspension, deletion - Authorization-by-logic: feature gates, role transitions, approval workflows - Quotas/limits: rate/usage limits, inventory, entitlements, seat licensing - Multi-tenant isolation: cross-organization data or action bleed - Event-driven flows: jobs, webhooks, sagas, compensations, idempotency ## High-Value Targets - Pricing/cart: price locks, quote to order, tax/shipping computation - Discount engines: stacking, mutual exclusivity, scope (cart vs item), once-per-user enforcement - Payments: auth/capture/void/refund sequences, partials, split tenders, chargebacks, idempotency keys - Credits/gift cards/vouchers: issuance, redemption, reversal, expiry, transferability - Subscriptions: proration, upgrade/downgrade, trial extension, seat counts, meter reporting - Refunds/returns/RMAs: multi-item partials, restocking fees, return window edges - Admin/staff operations: impersonation, manual adjustments, credit/refund issuance, account flags - Quotas/limits: daily/monthly usage, inventory reservations, feature usage counters ## Reconnaissance ### Workflow Mapping - Derive endpoints from the UI and proxy/network logs; map hidden/undocumented API calls, especially finalize/confirm endpoints - Identify tokens/flags: stepToken, paymentIntentId, orderStatus, reviewState, approvalId; test reuse across users/sessions - Document invariants: conservation of value (ledger balance), uniqueness (idempotency), monotonicity (non-decreasing counters), exclusivity (one active subscription) ### Input Surface - Hidden fields and client-computed totals; server must recompute on trusted sources - Alternate encodings and shapes: arrays instead of scalars, objects with unexpected keys, null/empty/0/negative, scientific notation - Business selectors: currency, locale, timezone, tax region; vary to trigger rounding and ruleset changes ### State and Time Axes - Replays: resubmit stale finalize/confirm requests - Out-of-order: call finalize before verify; refund before capture; cancel after ship - Time windows: end-of-day/month cutovers, daylight saving, grace periods, trial expiry edges ## Key Vulnerabilities ### State Machine Abuse - Skip or reorder steps via direct API calls; verify server enforces preconditions on each transition - Replay prior steps with altered parameters (e.g., swap price after approval but before capture) - Split a single constrained action into many sub-actions under the threshold (limit slicing) ### Concurrency and Idempotency - Parallelize identical operations to bypass atomic checks (create, apply, redeem, transfer) - Abuse idempotency: key scoped to path but not principal → reuse other users' keys; or idempotency stored only in cache - Message reprocessing: queue workers re-run tasks on retry without idempotent guards; cause duplicate fulfillment/refund ### Numeric and Currency - Floating point vs decimal rounding; rounding/truncation favoring attacker at boundaries - Cross-currency arbitrage: buy in currency A, refund in B at stale rates; tax rounding per-item vs per-order - Negative amounts, zero-price, free shipping thresholds, minimum/maximum guardrails ### Quotas, Limits, and Inventory - Off-by-one and time-bound resets (UTC vs local); pre-warm at T-1s and post-fire at T+1s - Reservation/hold leaks: reserve multiple, complete one, release not enforced; backorder logic inconsistencies - Distributed counters without strong consistency enabling double-consumption ### Refunds and Chargebacks - Double-refund: refund via UI and support tool; refund partials summing above captured amount - Refund after benefits consumed (downloaded digital goods, shipped items) due to missing post-consumption checks ### Feature Gates and Roles - Feature flags enforced client-side or at edge but not in core services; toggle names guessed or fallback to default-enabled - Role transitions leaving stale capabilities (retain premium after downgrade; retain admin endpoints after demotion) ## Advanced Techniques ### Event-Driven Sagas - Saga/compensation gaps: trigger compensation without original success; or execute success twice without compensation - Outbox/Inbox patterns missing idempotency → duplicate downstream side effects - Cron/backfill jobs operating outside request-time authorization; mutate state broadly ### Microservices Boundaries - Cross-service assumption mismatch: one service validates total, another trusts line items; alter between calls - Header trust: internal services trusting X-Role or X-User-Id from untrusted edges - Partial failure windows: two-phase actions where phase 1 commits without phase 2, leaving exploitable intermediate state ### Multi-Tenant Isolation - Tenant-scoped counters and credits updated without tenant key in the where-clause; leak across orgs - Admin aggregate views allowing actions that impact other tenants due to missing per-tenant enforcement ## Bypass Techniques - Content-type switching (JSON/form/multipart) to hit different code paths - Method alternation (GET performing state change; overrides via X-HTTP-Method-Override) - Client recomputation: totals, taxes, discounts computed on client and accepted by server - Cache/gateway differentials: stale decisions from CDN/APIM that are not identity-aware ## Special Contexts ### E-commerce - Stack incompatible discounts via parallel apply; remove qualifying item after discount applied; retain free shipping after cart changes - Modify shipping tier post-quote; abuse returns to keep product and refund ### Banking/Fintech - Split transfers to bypass per-transaction threshold; schedule vs instant path inconsistencies - Exploit grace periods on holds/authorizations to withdraw again before settlement ### SaaS/B2B - Seat licensing: race seat assignment to exceed purchased seats; stale license checks in background tasks - Usage metering: report late or duplicate usage to avoid billing or to over-consume ## Chaining Attacks - Business logic + race: duplicate benefits before state updates - Business logic + IDOR: operate on others' resources once a workflow leak reveals IDs - Business logic + CSRF: force a victim to complete a sensitive step sequence ## Testing Methodology 1. **Enumerate state machine** - Per critical workflow (states, transitions, pre/post-conditions); note invariants 2. **Build Actor × Action × Resource matrix** - Unauth, basic user, premium, staff/admin; identify actions per role 3. **Test transitions** - Step skipping, repetition, reordering, late mutation 4. **Introduce variance** - Time, concurrency, channel (mobile/web/API/GraphQL), content-types 5. **Validate persistence boundaries** - All services, queues, and jobs re-enforce invariants ## Validation 1. Show an invariant violation (e.g., two refunds for one charge, negative inventory, exceeding quotas) 2. Provide side-by-side evidence for intended vs abused flows with the same principal 3. Demonstrate durability: the undesired state persists and is observable in authoritative sources (ledger, emails, admin views) 4. Quantify impact per action and at scale (unit loss × feasible repetitions) ## False Positives - Promotional behavior explicitly allowed by policy (documented free trials, goodwill credits) - Visual-only inconsistencies with no durable or exploitable state change - Admin-only operations with proper audit and approvals ## Impact - Direct financial loss (fraud, arbitrage, over-refunds, unpaid consumption) - Regulatory/contractual violations (billing accuracy, consumer protection) - Denial of inventory/services to legitimate users through resource exhaustion - Privilege retention or unauthorized access to premium features ## Pro Tips 1. Start from invariants and ledgers, not UI—prove conservation of value breaks 2. Test with time and concurrency; many bugs only appear under pressure 3. Recompute totals server-side; never accept client math—flag when you observe otherwise 4. Treat idempotency and retries as first-class: verify key scope and persistence 5. Probe background workers and webhooks separately; they often skip auth and rule checks 6. Validate role/feature gates at the service that mutates state, not only at the edge 7. Explore end-of-period edges (month-end, trial end, DST) for rounding and window issues 8. Use minimal, auditable PoCs that demonstrate durable state change and exact loss 9. Chain with authorization tests (IDOR/Function-level access) to magnify impact 10. When in doubt, map the state machine; gaps appear where transitions lack server-side guards ## Summary Business logic security is the enforcement of domain invariants under adversarial sequencing, timing, and inputs. If any step trusts the client or prior steps, expect abuse.