I hope by the time you read this, Nessus will have fixed its installer, and this will be a non-issue. As of this writing, though, Nessus (Agent 10.9) currently delivers an “installer” that’s just a hard link to a hidden real installer.

The visible “installer” (Install Nessus Agent.pkg) is less than 100 KB in size (as it just points to the real installer):

But the invisible installer (.NessusAgent.pkg) is over 40 MB in size (as it’s a real installer):

If you try to install the visible “installer,” macOS will not let you:

sudo installer -pkg /Volumes/Nessus\ Agent\ Install/Install\ Nessus\ Agent.pkg -target /
installer: Package name is Tenable Nessus Agent
installer: Installing at base path /
2025-08-19 21:38:40.299 installer[53848:2565752] Opened package is not the same at install time...
installer: The install failed. (The Installer could not install the software because there was
no software found to install. The Installer could not install the software.)

Instead, you have to install the actual invisible package:

sudo installer -pkg /Volumes/Nessus\ Agent\ Install/.NessusAgent.pkg -target /
installer: Package name is .NessusAgent
installer: Installing at base path /
installer: The install was successful.

While this change can be frustrating if your workflow with Nessus is to install the visible pointer .pkg, it makes sense from a security standpoint for Apple to not follow links for installers.

It’s not clear why Nessus even has a fake installer hard-linked to a real hidden installer in the first place. Hopefully, they fix it soon so there’s only one real Nessus installer. In the meantime, the workaround is to use the actual hidden installer directly.