Kibana provides search and data visualization capabilities for data indexed in Elasticsearch. The service runs per default on port 5601. Kibana also acts as the user interface for monitoring, managing, and securing an Elastic Stack cluster.
Authentication in Kibana is linked to the credentials from Elasticsearch. If authentication is disabled in Elasticsearch, Kibana also should be accessible without credentials. Otherwise the same credentials valid for Elasticsearch should be working when logging in to Kibana. The rights of the users in Elasticsearch are the same as in Kibana.
You might find credentials in the configuration file /etc/kibana/kibana.yml. If those credentials are not for the user kibana_system, it should be tried to use them for accessing further data. They could have more rights then the kibana_system user, which only has access to the monitoring API and the .kibana index.
When having access to Kibana you can do several things:
Try to access data from Elasticsearch
Check if you can access the users panel and if you can edit, delete or create new users, roles or API Keys (Stack Management -> Users/Roles/API Keys)
Check the current version for vulnerabilities (There was a RCE vulnerability in 2019 for Kibana versions < 6.6.0 )
If SSL/TLS is not enabled, it should be evaluated, whether sensitive information can be leaked.