A vulnerability in Kibana could expose sensitive information related to Elastic Stack monitoring in the Kibana page source. Elastic Stack monitoring features provide a way to keep a pulse on the health and performance of your Elasticsearch cluster. Authentication with a vulnerable Kibana instance is not required to view the exposed information. The Elastic Stack monitoring exposure only impacts users that have set any of the optional monitoring.ui.elasticsearch.* settings in order to configure Kibana as a remote UI for Elastic Stack Monitoring. The same vulnerability in Kibana could expose other non-sensitive application-internal information in the page source.
https://discuss.elastic.co/t/kibana-7-17-3-and-8-1-3-security-update/302826