It was first brought forth to the public in 2009 by Stefano di Paola and Luca Carettoni, in the conference OWASP EU09 Poland.
[1] 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.
HPP could be used for cross channel pollution, bypassing CSRF protection and WAF input validation checks.
[3] Proper input validation and awareness about web technology on HPP is protection against HTTP Parameter Pollution.