By default, non-anonymous Confluence Knowledgebase (KB) articles require to be logged in.
This also applies for the customer portal:


The login prompt can be avoided by using the SAML Single Sign On app in Confluence and by adding the entry 

/plugins/servlet/remotepageview.*

to the Force SSO URLs section (SAML SSO configurations for Confluence → Redirection → Advanced redirection controls).

After saving the SAML SSO configuration, SSO should automatically trigger within the Customer Portal KB View.
As long as there is an active identity provider user session in the browser, no login is required either.
If not, the user needs to authenticate via the Identity Provider again to access the Customer Portal KB View.