HTTP parameter pollution

HTTP Parameter Pollution (HPP) is a web application vulnerability exploited by injecting encoded query string delimiters in already existing parameters. The vulnerability occurs if user input is not correctly encoded for output by a web application. This vulnerability allows the injection of parameters into web application-created URLs. It was first brought forth to the public in 2009 by Stefano di Paola and Luca Carettoni, in the conference OWASP EU09 Poland. The impact of such vulnerability varies, and it can range from "simple annoyance" to complete disruption of the intended behavior of a web application. Overriding HTTP parameters to alter a web application's behavior, bypassing input and access validation checkpoints, as well as other indirect vulnerabilities, are possible consequences of a HPP attack.

There is no RFC standard on what should be done when it has passed multiple parameters. HPP could be used for cross channel pollution, bypassing CSRF protection and WAF input validation checks.

Behaviour
When they are passed multiple parameters with the same name, here is how various back ends behave.

Client-side

 * First Order / Reflected HPP
 * Second Order / Stored HPP
 * Third Order / DOM HPP

Server-side

 * Standard HPP
 * Second Order HPP

Prevention
Proper input validation and awareness about web technology on HPP is protection against HTTP Parameter Pollution.