Security auditing is a chore that many system and network administrators tend to neglect. In today's IT climate of belt tightening, most administrators have a mandate to do more with fewer resources. Proactive security measures are often low on management's list of prioritiesuntil someone hacks into one of your servers. Nessus, an open source security assessment tool, offers a powerful solution for administrators who want to secure their environment before a security breach happens. Nessus is capable of identifying a dizzying array of potential vulnerabilities in your network, and producing detailed reports of all the problems it finds:
Description of the problem
Severity level of the problem
CVE and Bugtraq IDs for the problem
Recommended action to correct the problem
With more than 1,200 security tests built in (and new ones added daily), Nessus can identify vulnerabilities and recommend solutions for virtually any environment.
A word of caution: Nessus is a powerful tool. Some of the built-in auditing tools are capable of crashing vulnerable services or hosts. These "dangerous" plug-ins are clearly labeled, and Nessus disables them by default. I strongly urge you to experiment in a test environment before enabling these plug-ins for a scan of production systems. To further minimize the danger of unscheduled outages, restrict aggressive scanning of production machines to scheduled maintenance windows. Having said that, let me add that aggressive scanning is the best way to find and fix security holes, and should be one of your goals in deploying Nessus. Just be careful!