HTTP Parameter Pollution

HTTP Parameter Pollution (HPP) is a Web attack evasion technique that allows an attacker to craft a HTTP request in order to manipulate web logics or retrieve hidden information. This evasion technique is based on splitting an attack vector between multiple instances of a parameter with the same name (?param1=value&param1=value). As there is no formal way of parsing HTTP parameters, individual web technologies have their own unique way of parsing and reading URL parameters with the same name. Some taking the first occurrence, some taking the last occurrence, and some reading it as an array. This behavior is abused by the attacker in order to bypass pattern-based security mechanisms.

Summary

Tools
Methodology
- Parameter Pollution Table
- Parameter Pollution Payloads
References

Tools

Burp Suite: Manually modify requests to test duplicate parameters.
OWASP ZAP: Intercept and manipulate HTTP parameters.

Methodology

HTTP Parameter Pollution (HPP) is a web security vulnerability where an attacker injects multiple instances of the same HTTP parameter into a request. The server's behavior when processing duplicate parameters can vary, potentially leading to unexpected or exploitable behavior.

HPP can target two levels:

Client-Side HPP: Exploits JavaScript code running on the client (browser).
Server-Side HPP: Exploits how the server processes multiple parameters with the same name.

Examples:

/app?debug=false&debug=true
/transfer?amount=1&amount=5000

Parameter Pollution Table

When ?par1=a&par1=b

Technology	Parsing Result	outcome (par1=)
ASP.NET/IIS	All occurrences	a,b
ASP/IIS	All occurrences	a,b
Golang net/http - `r.URL.Query().Get("param")`	First occurrence	a
Golang net/http - `r.URL.Query()["param"]`	All occurrences in array	['a','b']
IBM HTTP Server	First occurrence	a
IBM Lotus Domino	First occurrence	a
JSP,Servlet/Tomcat	First occurrence	a
mod_wsgi (Python)/Apache	First occurrence	a
Nodejs	All occurrences	a,b
Perl CGI/Apache	First occurrence	a
Perl CGI/Apache	First occurrence	a
PHP/Apache	Last occurrence	b
PHP/Zues	Last occurrence	b
Python Django	Last occurrence	b
Python Flask	First occurrence	a
Python/Zope	All occurrences in array	['a','b']
Ruby on Rails	Last occurrence	b

Parameter Pollution Payloads

Duplicate Parameters:
```
param=value1&param=value2
```

Array Injection:

param[]=value1
param[]=value1&param[]=value2
param[]=value1&param=value2
param=value1&param[]=value2

Encoded Injection:
```
param=value1%26other=value2
```
Nested Injection:
```
param[key1]=value1&param[key2]=value2
```

JSON Injection:

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

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