Open-source vulnerability assessment tools are effective security 
scanners to detect missing patches, misconfiguration, and other 
vulnerabilities. These scanners also publish their code publicly and 
provide a free version with most, if not all, features. To help you 
select the best tool, we picked the top two tools in each of three 
categories: devices (endpoints, routers, containers, etc.), websites 
and applications (aka web and app), and specialty scanners for 
specific assets.

Here are the six best open-source vulnerability scanners:

    Nmap: Best device scanner overall
    OpenVAS: Best device scanner for user experience
    ZAP: Best web and app scanner overall 
    OSV-Scanner: Best web and app scanner for library dependency
    CloudSploit: Best specialty scanner for cloud and containers
    sqlmap: Best specialty scanner for databases

Reposted courtesy of esecurityplanet