"Free" means different things depending on which scanner you're looking at - genuinely free and open-source with no strings attached, or free-to-start with a paid plan once you need more. Here's an honest breakdown of what you actually get at zero cost from each.
OWASP ZAP - Free, Open-Source, Self-Hosted
The most complete free option with no catch: OWASP ZAP is fully open-source, actively maintained by the OWASP Foundation, and covers real OWASP Top 10 testing. The cost isn't hidden in the software - it's in the setup and maintenance. You host it yourself, configure scan policies yourself, and interpret raw findings yourself. For a team with the DevOps time and security background to run it well, there's no better zero-cost option.
Nikto - Free, CLI, Narrower Scope
A free command-line scanner focused on server misconfigurations and outdated software versions - useful for quick reconnaissance, but it predates modern web application vulnerability classes (it doesn't cover API security, GraphQL, or cloud storage exposure) and produces raw text output rather than a report you'd hand to anyone else.
Browser DevTools + Manual Checks - Free, Manual
A surprising amount of a basic security review is possible with nothing but curl and browser DevTools: checking security headers (curl -sI yoursite.com), inspecting cookie flags in the Application tab, and reviewing the Network tab for mixed content or exposed API responses. This doesn't scale past a handful of pages and finds nothing that requires actually probing for a vulnerability, but it's genuinely free and takes minutes.
Shieldome - Free to Start, Paid Beyond the Trial
Not free in the same sense as ZAP - Shieldome is a commercial SaaS product - but it's worth including because "free to start" is what a lot of people actually mean by this search. One full scan requires no credit card, giving you a real OWASP Top 10 report, security grade, and dark web credential check without any setup, hosting, or configuration. Continuing past that (scheduled monitoring, more domains) is a paid plan starting at $30/month.
Mozilla Observatory - Free, Narrow Scope
A free tool specifically for security headers and TLS configuration grading - genuinely useful for that narrow slice, not a general vulnerability scanner. Good as a quick first check, not a substitute for OWASP Top 10 coverage.
Which "Free" Actually Fits What You Need
| What you need | Best fit |
|---|---|
| Full OWASP coverage, willing to self-host and configure | OWASP ZAP |
| Quick server misconfiguration check | Nikto |
| Just security headers and TLS grade | Mozilla Observatory |
| One real report with zero setup, right now | Shieldome (free scan) |
| Ongoing automated monitoring | None of the above are free - this is a paid-tier decision |
Our guide to choosing a security scanner covers the evaluation criteria worth applying to any of these, free or paid. Our Shieldome vs OWASP ZAP comparison goes deeper on that specific tradeoff. Run your free scan - no card required.