HTTP Parameter Pollution Lets Attackers Bypass WAFs by Exploiting Framework Parsing Gaps

HTTP Parameter Pollution (HPP) is a technique where attackers exploit differences between how Web Application Firewalls (WAFs) inspect parameters and how backend frameworks actually process them. Because frameworks like PHP, Spring, and Django each resolve duplicate query parameters differently — using the first, last, or a concatenated value — an attacker can position a malicious payload exactly where the WAF does not evaluate it. A study by Ethiack found that the most sophisticated HPP payload achieved a 70.6% WAF bypass rate, and no tested WAF vendor blocked all attack scenarios. The vulnerability is structural, stemming from the separation between the inspection layer and the execution layer, and cannot be patched with a simple WAF signature update. A related critical vulnerability, CVE-2025-7783 (CVSS 9.4), was recently disclosed in the Node.js form-data library, where predictable multipart boundaries enabled parameter injection, affecting versions below 2.5.4, 3.0.0–3.0.3, and 4.0.0–4.0.3.