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 on unique way of parsing and reading URL parameters with the same name. Some taking the first occurance, some taking the last occurance, and some reading it as an array. This behavior is abused by the attacker in order to bypass pattern-based security mechanisms.


No tools needed. Maybe Burp or OWASP ZAP.

How to test

HPP allows an attacker to bypass pattern based/black list proxies or Web Application Firewall detection mechanisms. This can be done with or without the knowledge of the web technology behind the proxy, and can be achieved through simple trial and error.

Example scenario.
WAF - Reads first param
Origin Service - Reads second param. In this scenario, developer trusted WAF and did not implement sanity checks.

Attacker --' OR 1=1;## --> WAF (reads first 'search' param, looks innocent. passes on) --> Origin Service (reads second 'search' param, injection happens if no checks are done here.)

Table of refence for which technology reads which parameter

When ?par1=a&par1=b
| Technology | Parsing Result |outcome (par1=)|
| —————— |————— |:————-:|
| ASP.NET/IIS |All occurrences |a,b |
| ASP/IIS |All occurrences |a,b |
| PHP/Apache |Last occurrence |b |
| PHP/Zues |Last occurrence |b |
| JSP,Servlet/Tomcat |First occurrence |a |
| Perl CGI/Apache |First occurrence |a |
| Python Flask |First occurrence |a |
| Python Django |Last occurrence |b |
| Nodejs |All occurrences |a,b |
| Golang net/http - r.URL.Query().Get("param") |First occurrence |a |
| Golang net/http - r.URL.Query()["param"] |All occurrences |a,b |