Zed Attack Proxy is a free and open-source penetration test tool that is being maintained under the wing of the Open Web Application Security Project. ZAP is flexible and extensible and was specifically designed for testing web applications. ZAP is a "man in the middle proxy" that acts as a firewall between the browser and the web app. It can intercept and inspect the messages between the browser and web applications, modify them if necessary, and then forward those packets to the destination. It can be used both as a standalone application and as a daemon process. ZAP offers functionality for all skill levels, from developers to security testers, to security specialists, to security testers who are new to security testing. ZAP supports all major OSes and Dockers, so you don't have to stick with one OS. You can access additional functionality from the ZAP Marketplace by downloading add-ons.