@fafaba In your knowledge base article you can click the More dropdown>Control Audience access>Private - registration required>then select a list of allowable contacts
We also have a VAST Knowledge Base where we don’t want all the articles to be visible in Navigation, but we want them to be searchable and accessible to users without having to set permissions. One workaround that I have found is creating a category that isn’t visible and adding the articles that you want accessible (but invisible) to that category.