"The open-source ESAPI WAF is a departure from commercial, network-based firewalls, as well as ModSecurity's free WAF, says Arshan Dabirsiaghi, developer of the ESAPI WAF and director of research for Aspect Security. Dabirsiaghi will roll out the WAF at the OWASP Conference in Washington, D.C., in November.

"WAFs today are deployed as appliances meant to protect a suite of applications. They're kind of part of your network and not part of your application," Dabirsiaghi says. "This is really an application-layer WAF rather than a network or appliance-layer WAF...it lives inside your application so you have a lot more control as an application owner [who's] enforcing rules."

The ESAPI WAF isn't the first open-source (or free) WAF: ModSecurity, which is also a software-based WAF, was one of the first freebies. The ModSecurity WAF, which is an Apache module, runs outside the application on the server." - Darkreading