What is a WAF?

Context

In a typical scalable web infrastructure, a proxy will place in front of the backend. The proxy is forwarding a request to the backend. It can be a load balancer, a caching server, or simply a reverse proxy. A firewall functionality can be enabled on each of those.

Purpose

The purpose of the Web Application Firewall is to filters, monitors, and blocks incoming and/or outgoing HTTP traffic.

Analysis of incoming traffic can be to identify attack patterns to block or logged those.

Vendors

There are numerous WAF softwares that can be used. Many of those are included in cloud infrastructure.

The two main open-source projects are modsecurity and naxsi.