Lynis Security Auditor
ToolLynis is a free, open-source security auditor from CISOfy for Linux, macOS and other Unix systems. Nothing to install: extract the archive, run lynis audit system, and hundreds of tests walk the server and point out where hardening is missing. It makes a quick before-and-after check for any baseline, which is the part auditors like and the part you will actually rerun.
Resources
Videos
Linux Security Auditing with Lynis: Step-by-Step Guide
Ten minutes on a Debian-family box with Lynis 3.0.9: sudo apt install lynis, then the first audit, with green, red and yellow brackets marking passes, warnings and suggestions. At 4:33 the hardening index reads 63, and the host says plainly that it needs work. The last section is the one to copy: a cron entry that runs /usr/bin/lynis audit system at 2 a.m., so the score gets checked whether anyone remembers. Line this month's number up against last month's and the trend says more than any single run.
How Secure Is Your Linux Server? Test It with This Free Audit Tool
Kyle from Cisco U installs Lynis on a base Linux install and runs sudo lynis audit; at 8:12 the hardening index comes back 59, with fail2ban near the top of the suggestions. Then the pair add fail2ban, tighten SSH and switch on UFW, which the status check showed was off. The rescan at 15:50 reads 60. One point for three fixes looks stingy, and that is the lesson: the index rewards a long list of small changes, so treat it as a trend line and keep chipping.