If you are starting the process over, ensure you follow these steps precisely to avoid future errors:
When setting up Tenable Nessus in an air-gapped or offline environment, you must pair your Challenge Code (generated locally by your scanner) with your Activation Code (provided by Tenable) on an internet-connected computer.
Go to your offline server terminal or command line to fetch the current machine fingerprint. /opt/nessus/sbin/nessuscli fetch --challenge Use code with caution. Windows System:
: On a machine with internet access, go to the Nessus Offline Registration Page and enter your Challenge Code and Activation Code . nessus offline activation license already in use full
It was coming from the scanner itself.
Open an administrator command prompt, navigate to the Nessus installation directory (typically C:\Program Files\Tenable\Nessus\ ), and run:
Copy the generated 40-character alphanumeric string. Keep this terminal window open. Step 4: Perform the Offline Registration Exchange If you are starting the process over, ensure
Download the appropriate installation package for your operating system from the Tenable Downloads page. Perform the installation using the standard procedure for your platform:
Log into the official Tenable Community Portal using an internet-connected device. Click on located in the top navigation panel.
: On the offline machine, run the following command to get your unique challenge code: Linux : /opt/nessus/sbin/nessuscli fetch --challenge Windows System: : On a machine with internet
: /opt/nessus/sbin/nessuscli fetch --register-offline nessus.license
Navigate to the (or the Tenable Enterprise Cloud portal if using Nessus Expert/Professional via enterprise accounts). Log in with your registered credentials. Navigate to the My Products or Products section.
Step-by-Step Resolution: Resetting Your Nessus Activation Code
When you activate Nessus offline, the Tenable plugins website links your specific to a unique Challenge String generated by your host machine. If you attempt to use that same activation code again—even on the exact same machine after a reinstall, hardware change, or virtual machine (VM) snapshot restoration—the Tenable licensing server sees it as a duplicate installation attempt on a second device. Because standard Nessus Professional licenses only allow one active installation at a time, the server blocks the request. Common triggers for this error include: Reinstalling the operating system on the Nessus host.