Overview What is a Web Application Firewall? A web application firewall (WAF) is a specific form of application firewall that filters, monitors, and blocks HTTP traffic to and from a web service. By inspecting HTTP traffic, it can prevent attacks exploiting a web application’s known vulnerabilities, such as SQL injection, cross-site scripting (XSS), file inclusion, and improper system configuration. 1 Many basic security measures are applied in a WAF are supported by the Open Web Application Security Project (OWASP) owasp.