Is there a free open-source alternative to Nessus?

Yes. ShadowSecurityScanner is a free, MIT-licensed network vulnerability scanner and penetration testing tool. Unlike Nessus Essentials it has no 16-IP scan limit, needs no account or activation, and runs fully offline as a single desktop app on Windows, macOS and Linux. OpenVAS/GVM is another open-source option, though it requires a multi-component Linux server.

What is the difference between Nessus Essentials and ShadowSecurityScanner?

Nessus Essentials is a free tier of the proprietary Tenable Nessus, capped at 16 IP addresses and requiring an activation account. ShadowSecurityScanner is fully open source (MIT), has no IP cap, requires no account, sends no telemetry, and ranks findings by EPSS exploit probability and CISA KEV status out of the box.

Can I migrate my Nessus workflow to ShadowSecurityScanner?

Most workflows map directly: define targets, run authenticated or unauthenticated scans, triage by exploitability and export a report. ShadowSecurityScanner adds SARIF export for CI pipelines and scan diffing to track new, regressed and resolved findings between runs.

Is ShadowSecurityScanner really free for commercial use?

Yes. It is released under the permissive MIT license, so it is free for personal, educational and commercial use, with no per-IP or per-seat charges.

Nessus alternative

A free, open-source Nessus alternative

Network vulnerability scanning without the 16-IP cap, the account or the cloud · 2026

Nessus (by Tenable) is the best-known commercial network vulnerability scanner, but its free Nessus Essentials tier is capped at 16 IP addresses, requires an activation account, and the full product is a paid subscription. If you want a genuinely free, open-source Nessus alternative with no scan limit, ShadowSecurityScanner is built for exactly that: an MIT-licensed desktop scanner that runs offline and ranks findings by real-world exploitability.

Why teams look for a Nessus alternative

The 16-IP limit — Nessus Essentials can't cover a real network segment.
Cost — the paid tier is a recurring per-scanner subscription.
Account & activation — you need a Tenable account and an online activation step.
Closed source — you can't audit, self-host freely or extend the engine.
Prioritisation — a flat, CVSS-sorted list buries the vulnerabilities attackers actually use.

ShadowSecurityScanner vs Nessus at a glance

Capability	ShadowSecurityScanner	Nessus (Essentials / Pro)
License	Open source (MIT)	Proprietary
Price	Free, unlimited	Free tier (16 IPs) / paid Pro
IP / target limit	None	16 IPs on Essentials
Account required	No	Yes (activation)
Deployment	Single desktop binary	Local service + web UI
Cloud / telemetry	None — fully offline	Account & activation
EPSS exploit scoring	Built in	Partial
CISA KEV flagging	Built in	Partial
Scan diffing	New / regressed / resolved	Limited
SARIF export (CI)	Yes	No
Platforms	Windows · macOS · Linux	Windows · macOS · Linux

What you get instead

ShadowSecurityScanner does the core job of Nessus — port scanning, service and OS fingerprinting, and thousands of catalogued network and web checks — then adds the part that saves the most time: exploit-aware prioritisation. Every finding carries its FIRST.org EPSS exploit probability and a CISA KEV flag, sorted KEV → EPSS → severity, so you fix what attackers actually exploit first instead of chasing every CVSS 9.8. It exports PDF, HTML, SARIF, XML and CSV, and scan diffing shows exactly what changed between runs.

When Nessus is still the right call

Nessus is a mature product with a very large plugin library, deep credentialed-scan and compliance coverage, and commercial support contracts. If you need vendor SLAs and broad regulatory auditing at enterprise scale and have the budget, it remains a strong choice. For individual pentesters, consultants, sysadmins and small teams who want a free, private, no-limits tool, ShadowSecurityScanner is the pragmatic alternative. See the full three-way comparison vs Nessus and OpenVAS.

Try the free, open-source alternative

Download ShadowSecurityScanner for Windows, macOS or Linux — a single binary, no installer, no account.

Download ShadowSecurityScanner

A free alternative to OpenVAS / GVM.
ShadowSecurityScanner vs Nessus vs OpenVAS — full comparison.
How to run a network vulnerability scan.
Best free open-source penetration testing tools.

Comparison reflects publicly documented features at the time of writing and is for orientation only; verify current capabilities with each vendor. Nessus is a trademark of Tenable, Inc. Product names belong to their respective owners.