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.