SKILL: Business Logic Vulnerabilities — Expert Attack Playbook

AI LOAD INSTRUCTION: Business logic flaws are scanner-invisible and high-reward on bug bounty. This skill covers race conditions, price manipulation, workflow bypass, coupon/referral abuse, negative values, and state machine attacks. These require human reasoning, not automation. For specific exploitation techniques (payment precision/overflow, captcha bypass, password reset flaws, user enumeration), load the companion SCENARIOS.md.

Extended Scenarios

Also load SCENARIOS.md when you need:

• Payment precision & integer overflow attacks — 32-bit overflow to negative, decimal rounding exploitation, negative shipping fees
• Payment parameter tampering checklist — price, discount, currency, gateway, return_url fields
• Condition race practical patterns — parallel coupon application, gift card double-spend with Burp group send
• Captcha bypass techniques — drop verification request, remove parameter, clear cookies to reset counter, OCR with tesseract
• Arbitrary password reset — predictable tokens (md5(username)), session replacement attack, registration overwrite
• User information enumeration — login error message difference, masked data reconstruction across endpoints, base64 uid cookie manipulation
• Frontend restriction bypass — array parameters for multiple coupons (couponid[0]/couponid[1]), remove disabled/readonly attributes
• Application-layer DoS patterns — regex backtracking, WebSocket abuse

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1. PRICE AND VALUE MANIPULATION

Negative Quantity / Price

Many applications validate "amount > 0" but not for currency:

Add to cart with quantity: -1
Update quantity to: -100
{
  "quantity": -5,
  "price": -99.99     ← may be accepted
}

Impact: Receive credit to account, items for free, bank transfers in reverse.

Integer Overflow

quantity: 2147483648   ← INT_MAX + 1 overflows to negative in 32-bit
price: 9999999999999   ← exceeds float precision → rounds to 0

Rounding Manipulation

Item price: $0.001
Order 1000 items → each rounds down → total = $0.00

Currency Exchange Rate Lag

1. Deposit using currency A at rate X
2. Rate changes
3. Withdraw using currency A at new rate → profit from rate difference

Free Upgrade via Promo Stacking

Test combining discount codes, referral credits, welcome bonuses:

Apply promo: FREE50  → 50% off
Apply promo: REFER10 → additional 10%
Apply loyalty points → additional discount
Total: -$5 (free + credit)

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2. RACE CONDITIONS

Concept: Two operations run simultaneously before the first completes its check-update cycle.

Double-Spend / Double-Redeem

# Send same request simultaneously (~millisecond apart):
# Use Burp Repeater "Send to Group" or Race Conditions tool:

POST /api/use-coupon    ← send 20 parallel requests
POST /api/redeem-gift   ← same coupon code, parallel
POST /api/withdraw-funds ← same balance, parallel

# If check and update are non-atomic:
# Thread 1: check(balance >= 100) → TRUE
# Thread 2: check(balance >= 100) → TRUE (before Thread 1 deducted)
# Thread 1: balance -= 100
# Thread 2: balance -= 100 → BOTH succeed → double-spend

Race Condition Test with Burp Suite

1. Capture request
2. Send to Repeater → duplicate 20+ times
3. "Send group in parallel" (Burp 2023+)
4. Check: did any duplicate succeed?

Account Registration Race

Register with same email simultaneously → two accounts created → data isolation broken
Password reset token race → reuse same token twice
Email verification race → verify multiple email addresses

Limit Bypass via Race

"Claim once" discounts, freebies, "first order" bonus:
→ Send 10 parallel POST /claim requests
→ Race window: all pass the "already claimed?" check before any write

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3. WORKFLOW / STEP SKIP BYPASS

Payment Flow Bypass

Normal flow:
  1. Add to cart
  2. Enter shipping info
  3. Enter payment (card/wallet)
  4. Click confirm → payment charged
  5. Order confirmed

Attack: Skip to step 5 directly
POST /api/orders/confirm {"cart_id": "1234", "payment_status": "paid"}
→ Does server trust client-sent payment_status?

Multi-Step Verification Skip

Password reset flow:
  1. Enter email
  2. Receive token
  3. Enter token
  4. Set new password (requires valid token from step 3)

Attack: Try going to step 4 without completing step 3:
POST /reset/password {"email": "victim@x.com", "token": "invalid", "new_pass": "hacked"}
→ Does server check that token was properly validated?

Or: Try token from old/expired flow → still accepted?

2FA Bypass

Normal flow:
  1. Enter username + password → success
  2. Enter 2FA code → logged in

Attack: After step 1 success, go directly to /dashboard
→ Is session created before 2FA completes?
→ Does /dashboard require 2FA-complete check or just "authenticated" flag?

Shipping Without Payment

  1. Add item to cart
  2. Enter shipping address
  3. Select payment method (credit card)
  4. Apply promo code (100% discount or gift card)  
  5. Final amount: $0
  6. Order placed

Attack: Apply 100% discount code → no actual payment processed → item ships

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4. COUPON AND REFERRAL ABUSE

Coupon Stacking

Test: Can you apply multiple coupon codes?
Test: Does "SAVE20" + promo stack to >100%?
Test: Apply coupon, remove item, keep discount applied, add different item

Referral Loop

1. Create Account_A
2. Register Account_B with Account_A's referral code → both get credit
3. Create Account_C with Account_B's referral code
4. Ad infinitum with throwaway emails
→ Infinite credit generation

Coupon = Fixed Dollar Amount on Variable-Price Item

Coupon: -$5 off any order
Buy item worth $3, use -$5 coupon → net -$2 (credit balance)

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5. ACCOUNT / PRIVILEGE LOGIC FLAWS

Email Verification Bypass

1. Register with email A (legitimate, verified)
2. Change email to B (attacker's email, unverified)
3. Use account as verified — does server enforce re-verification?

Or: Change email to victim's email → no verification → account claim

Password Reset Token Binding

1. Request password reset for your account → get token
2. Change your email address (account settings)
3. Reuse old password reset token → does it still work for old email?

Or: Request reset for victim@target.com
    Token sent to victim but check: does URL reveal predictable token pattern?

OAuth Account Linking Abuse

1. Have victim's email (but not their password)
2. Register with victim's email → get account with same email
3. Link OAuth (Google/GitHub) to your account
4. Victim logs in with Google → server finds email match → merges with YOUR account

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6. API BUSINESS LOGIC FLAWS

Object State Manipulation

order.status = "pending"
→ PUT /api/orders/1234 {"status": "refunded"}   ← self-trigger refund
→ PUT /api/orders/1234 {"status": "shipped"}    ← mark as shipped without shipping

Transaction Reuse

1. Initiate payment → get transaction_id
2. Complete purchase
3. Reuse same transaction_id for second purchase:
   POST /api/checkout {"transaction_id": "USED_TX", "cart": "new_cart"}

Limit Count Manipulation

Daily transfer limit = $1000
→ Transfer $999, cancel, transfer $999 (limit not updated on cancel)
→ Parallel transfers (race condition on limit check)
→ Different payment types not sharing limit counter

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7. SUBSCRIPTION / TIER CONFUSION

Free tier: cannot access feature X
Paid tier: can access feature X

Attack: 
- Sign up for paid trial → enable feature X → downgrade to free
  → Does feature X get disabled on downgrade? 
  → Can you continue using feature X?

Or:
- Inspect premium endpoint list from JS bundle
- Directly call premium endpoints with free account token
→ Server checks subscription for UI but not API?

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8. FILE UPLOAD BUSINESS LOGIC

For the full upload attack workflow beyond pure logic flaws, also load:

• upload insecure files

Upload size limit: 10MB
→ Upload 10MB → compress client-side → server decompresses → bomb?
(Zip bomb: 1KB zip → 1GB file = denial of service)

Upload type restriction:
→ Upload .csv for "data import" → inject formulas: =SYSTEM("calc")
  (CSV injection in Excel macro context)
→ Upload avatar → server converts → attack converter (ImageMagick, FFmpeg CVEs)

Storage path prediction:
→ /uploads/USER_ID/filename
→ Can you overwrite other user's file by knowing their ID + filename?

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9. TESTING APPROACH

For each business process:
1. Map the INTENDED flow (happy path)
2. Ask: "What if I skip step N?"
3. Ask: "What if I send negative/zero/MAX values?"
4. Ask: "What if I repeat this step twice?" (idempotency)
5. Ask: "What happens if I do A then B instead of B then A?"
6. Ask: "What if two users do this simultaneously?"
7. Ask: "Can I modify the 'trusted' status fields?"
8. Think from financial/resource impact angle → highest bounty

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10. HIGH-IMPACT CHECKLISTS

E-commerce / Payment

□ Negative quantity in cart
□ Apply multiple conflicting coupons
□ Race condition: double-spend gift card
□ Skip payment step directly to order confirmation  
□ Refund without return (trigger refund on delivered item via state change)
□ Currency rounding exploitation

Authentication / Account

□ 2FA bypass by direct URL access after password step
□ Password reset token reuse after email change
□ Email verification bypass (change email after verification)
□ OAuth account takeover via email match
□ Register with existing unverified email

Subscriptions / Limits

□ Access premium features after downgrade
□ Exceed rate/usage limits via parallel requests
□ Referral loop for infinite credits
□ Free trial ≠ time-limited (no enforcement after trial)
□ Direct API call to premium endpoint without subscription check