hunt-business-logic

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.

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• Shopify storefronts with draft/unpublished channel pages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.

1. 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?

1. 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.

1. 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.

1. 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.

1. 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.

1. 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|internal|staff|summit|admin)"

# Grep JS bundles for internal paths
curl -s https://target.com/assets/app.js | grep -oE '"/[a-zA-Z0-9/_-]{3,50}"' | sort -u

Email verification bypass:

# Access monitoring/protected features directly without completing verification
curl -s https://monitor.target.com/dashboard \
  -H "Cookie: session=<your_session>" \
  # Try skipping directly to the post-verification endpoint
  
# Replay verification token on different account
curl -X POST https://target.com/verify \
  -d 'token=VALID_TOKEN_FROM_ACCOUNT_A&email=account_b@example.com'

Grep patterns for client-side logic issues:

# Find price calculations in JS
grep -iE "(price|amount|total|cost)\s*[=*+]" app.js

# Find internal URLs in JS bundles
grep -oE '"/(employee|internal|staff|admin|summit)[^"]*"' *.js

# Find unvalidated IP header usage in server code
grep -iE "x-forwarded-for|x-real-ip|cf-connecting-ip" src/ -r

Common Root Causes

1. Server trusts client-supplied data for financial decisions. Developers offload price calculation to the frontend or pass amount fields through forms/URLs without re-validating on the server against a canonical source (the product database).

1. Verification is enforced only in the UI, not the API. Frontend hides features behind a verification gate, but the backend API endpoints are fully functional without a verified status — any authenticated request succeeds.

1. IP-based rate limiting reads from spoofable headers. Developers implement rate limits using request.headers['X-Forwarded-For'] instead of the actual connection IP, allowing trivial bypass by header manipulation.

1. Payment webhooks lack signature validation. Developers implement "success" webhooks without verifying the HMAC signature provided by the payment provider, allowing anyone to POST a fake success notification.

1. Internal/employee pages aren't access-controlled. Internal tools are deployed to production domains without authentication middleware, either because developers assume obscurity (unlisted URL) or forgot to apply auth to a new route.

1. Phone/callback verification is advisory, not enforced. Systems accept a phone number and grant trust to whoever submitted it, without confirming the submitter owns or controls that number.

1. Draft/channel-specific storefronts inherit full order functionality. Platforms like Shopify allow creating storefronts for specific channels (employee events) that are unlisted but still fully functional for order placement if the URL is known.

Bypass Techniques