/hunt-business-logic

name: hunt-business-logic
description: Hunting skill for business logic vulnerabilities. Built from 12 public bug bounty reports. Covers coupon-race-stacking (Instacart, Stripe, Reverb), negative-quantity-in-cart price tampering (Upserve, Eternal/Zomato), decimal/fraction price-field overflow (Shipt), client-side checkout amount trust on PayPal redirect (WordPress.org), price-per-unit mass-assignment (Krisp), and archived-price swap / cart-TOCTOU (Stripe). Use when hunting business logic — heavy emphasis on financial-impact-demonstrated cases.
sources: hackerone_public, github
report_count: 25

Crown Jewel Targets

Business logic vulnerabilities pay highest in platforms where financial transactions, identity verification, and access controls intersect with real-world consequences. The richest targets are:

• **E-commerce & payment platforms** (Valve/Steam, Shopify) — payment flow manipulation, free goods, price tampering
• **Marketplace & gig economy apps** (Airbnb, Uber) — identity/verification bypass enabling fraud or unsafe interactions
• **SaaS with tiered access** (Mozilla Monitor) — bypassing verification to unlock monitoring features without entitlement
• **High-traffic consumer apps** (Snapchat, Yelp) — rate-limit bypass enabling spam, enumeration, or abuse at scale

Asset types that pay: checkout flows, subscription endpoints, callback/verification systems, webhook handlers, employee/internal portals exposed to the internet, and any endpoint that trusts client-supplied data to make authorization decisions.

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Attack Surface Signals

**URL patterns to watch:**

• `/checkout`, `/order`, `/subscribe`, `/payment`, `/verify`, `/confirm`, `/callback`
• `/internal`, `/employee`, `/summit`, `/staff`, `/admin` — internal pages accidentally public
• `/api/v*/payment`, `/api/v*/notify`, `/webhook` — payment provider callbacks
• Endpoints accepting `X-Forwarded-For`, `X-Real-IP`, `CF-Connecting-IP` headers

**Response/header signals:**

• `Set-Cookie` with unvalidated session state tied to cart or order data
• Payment provider names in responses: `Smart2Pay`, `Stripe`, `PayPal`, `Braintree`
• Redirect chains through third-party payment pages (in-flight data opportunity)
• `200 OK` on subscription/verification endpoints with no CAPTCHA or token

**JS patterns:**

• Hardcoded internal URLs in frontend bundles (`/employee/`, `/staff/`, `/internal/`)
• Client-side price calculation before server submission
• Verification logic that only checks on the frontend (`if (verified) { ... }`)
• `fetch('/api/subscribe', { method: 'POST', body: ... })` with no anti-CSRF token or rate-limit token

**Tech stack signals:**

• Shopify storefronts with draft/unpublished channel pages
• Apps using IP-based rate limiting without session/account binding
• Payment webhooks with no HMAC signature validation
• SMS/phone callback flows that don't verify ownership before enabling features

---

Step-by-Step Hunting Methodology

1. **Map all authentication boundaries.** Spider the target. Identify pages/endpoints that serve authenticated content (employee portals, premium features, order pages) and test each unauthenticated. Look for internal pages indexed in JS bundles or linked from robots.txt/sitemap.xml.

2. **Identify every verification flow.** Enumerate: email verification, phone/SMS verification, payment verification, CAPTCHA, age gates. For each, test: what happens if you skip the verification step entirely? What happens if you replay a valid token on a different account?

3. **Test rate-limiting controls on every form.** For every POST endpoint (subscribe, login, OTP, search), send 50+ rapid requests. Vary: remove cookies, rotate `X-Forwarded-For` / `X-Real-IP` headers, change `User-Agent`. Check if the server uses IP from headers rather than connection IP.

4. **Intercept and tamper with payment flows.** Use Burp Suite to intercept every request between your browser, the application, and the payment provider. Identify where price, currency, order ID, or status fields are set. Attempt to modify amounts to $0.01 or currency to a low-value currency. Look for POST-back/webhook endpoints that accept payment confirmation — test if they validate HMAC/signature.

5. **Test phone/callback number verification.** Whenever a platform accepts a callback number, test: can you set it to a number you don't own? Does the platform call/text that number and grant trust based solely on submission? Try setting it to a victim's number.

6. **Check for unprotected employee/internal surfaces.** Search Shodan, GitHub, JS bundles, and Wayback Machine for internal subdomain/path references. Test access without authentication. Check if these surfaces allow order placement, data access, or privilege escalation.

7. **Validate business impact.** For each finding, determine: does this result in financial loss, unauthorized access, or data exposure? Document the end-to-end chain.

---

Payload & Detection Patterns

**Rate limit bypass via header rotation:**

# Rotate X-Forwarded-For to bypass IP rate limiting
for i in $(seq 1 100); do
  curl -s -X POST https://target.com/api/subscribe \
    -H "X-Forwarded-For: 10.0.0.$i" \
    -H "X-Real-IP: 10.0.0.$i" \
    -H "Content-Type: application/json" \
    -d '{"email":"victim+'"$i"'@example.com"}' \
    -o /dev/null -w "%{http_code}\n"
done

**Payment tampering — modify in-flight price:**

POST /payment/initiate HTTP/1.1
Host: target.com

amount=0.01&currency=USD&order_id=12345&product_id=99

# Look for unvalidated webhook endpoints
curl -X POST https://target.com/payment/callback \
  -H "Content-Type: application/json" \
  -d '{"status":"success","amount":"0.01","order_id":"12345","transaction_id":"fake-txn"}'

**Unauthenticated internal page discovery:**

# Check robots.txt and sitemap for internal paths
curl -s https://target.com/robots.txt | grep -iE "(disallow|allow)" 
curl -s https://target.com/sitemap.xml | grep -iE "(employee|in