Method 1

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

The login prompt can be avoided by using the SAML Single Sign-On app in Confluence and by adding the following entry to the Force SSO URLs section (SAML SSO configuration in 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.

Method 2

As of August 2021, it seems that with newer Jira & Confluence versions and with certain configurations a different configuration is required.
So far this was the case if the following conditions are true

  1. Application Links between Jira and Confluence are configured
  2. Application Links are using OAuth with impersonation (requires the same set of users and usernames in Jira/ Confluence)

These application links seem to take care of authenticating the users when they view the knowledge base articles in Jira.
The important thing with this kind of setup is to not add the entry describes in method 1 to the SAML SSO configuration because it will cause problems instead.
If you are experiencing problems with the knowledge base integration and 1 & 2 are true please remove the entry again in the SAML SSO configuration in Confluence → Redirection → Advanced redirection controls.