What is metasploitable3?

Metasploitable3 is a virtual machine built specifically to contain a large number of security vulnerabilities. A virtual machine is a software-based computer that runs inside your real computer, isolated from your actual system. This one is designed to be attacked on purpose, making it a safe practice target for learning how security testing tools work. The primary use case is as a target for Metasploit, a widely used security testing framework that helps researchers and security professionals find and test vulnerabilities in systems. Instead of testing against real production systems (which would be illegal and harmful), you run Metasploitable3 in a private environment and practice against it. Two versions of the vulnerable VM are available: one based on Windows Server 2008, and one based on Ubuntu 14.04 (an older version of Linux). You can download pre-built images using a tool called Vagrant, which manages virtual machine setup, or you can build your own copies from scratch using build scripts included in the repository. Building from scratch requires tools called Packer and Vagrant, plus a virtual machine platform like VirtualBox. The whole build needs about 65 GB of disk space and 4.5 GB of RAM. The repository includes build scripts for both Windows and Linux host machines. The default login credentials for the resulting VMs are the username and password "vagrant", and the full list of intentional vulnerabilities is documented on the project's wiki page. This project is intended strictly for security education and authorized testing. Running it inside an isolated local network, not connected to the public internet, is the expected and safe way to use it. It is released under a BSD-style open source license.