There are thousands of open source security tools with both defensive and offensive security capabilities.
The following are 10 essential security tools that will help you to secure your systems and networks. These open source security tools have been given the essential rating due to the fact that they are effective, well supported and easy to start getting value from.
1. Nmap - map your network and ports with the number one port scanning tool. Nmap now features powerful NSE scripts that can detect vulnerabilities, misconfiguration and security related information around network services. After you have nmap installed be sure to look at the features of the included ncat - its netcat on steroids.
2. OpenVAS - open source vulnerability scanning suite that grew from a fork of the Nessus engine when it went commercial. Manage all aspects of a security vulnerability management system from web based dashboards. For a fast and easy external scan with OpenVAS try our online OpenVAS scanner.
3. OSSEC - host based intrusion detection system or HIDS, easy to setup and configure. OSSEC has far reaching benefits for both security and operations staff.
4. Security Onion - a network security monitoring distribution that can replace expensive commercial grey boxes with blinking lights. Security Onion is easy to setup and configure. With minimal effort you will start to detect security related events on your network. Detect everything from brute force scanning kids to those nasty APT's.
5. Metasploit Framework - test all aspects of your security with an offensive focus. Primarily a penetration testing tool, Metasploit has modules that not only include exploits but also scanning and auditing.
6. OpenSSH - secure all your traffic between two points by tunnelling insecure protocols through an SSH tunnel. Includes scp providing easy access to copy files securely. Can be used as poor mans VPN for Open Wireless Access points (airports, coffee shops). Tunnel back through your home computer and the traffic is then secured in transit. Access internal network services through SSH tunnels using only one point of access. From Windows, you will probably want to have putty as a client and winscp for copying files. Under Linux just use the command line ssh and scp.
7. Wireshark - view traffic in as much detail as you want. Use Wireshark to follow network streams and find problems. Tcpdump and Tshark are command line alternatives. Wireshark runs on Windows, Linux, FreeBSD or OSX based systems.
8. BackTrack - an Ubuntu based Linux distribution that is configured with hundreds of security testing tools and scripts. Backtrack is well known with penetration testers and hobbyists alike.
9. Nikto - a web server testing tool that has been kicking around for over 10 years. Nikto is great for firing at a web server to find known vulnerable scripts, configuration mistakes and related security problems. It won't find your XSS and SQL web application bugs, but it does find many things that other tools miss. To get started try the Nikto Tutorial or the online hosted version.
10. Truecrypt - encrypt all the things. Truecrypt is a strong encryption utility that can encrypt entire volumes or create an encrypted container within a file system. Use Truecrypt to protect your flash drives. If it gets lost, even the NSA will have trouble reading the data.