HTTP Parameter Pollution (HPP) is a web attack technique where an attacker manipulates a URL or form data by injecting multiple parameters with the same name. This can confuse how a web application processes user input, potentially allowing the attacker to bypass security checks, access unauthorized data, or cause unexpected behavior. HPP exploits inconsistencies in how different web servers, frameworks, or backend systems handle duplicate parameters, making it a subtle but effective method for exploiting vulnerabilities in web applications.