Authentication
By default Elasticsearch doesn't have authentication enabled, so by default you can access everything inside the database without using any credentials.
You can verify that authentication is disabled with a request to:
curl -X GET "ELASTICSEARCH-SERVER:9200/_xpack/security/user"
{"error":{"root_cause":[{"type":"exception","reason":"Security must be explicitly enabled when using a [basic] license. Enable security by setting [xpack.security.enabled] to [true] in the elasticsearch.yml file and restart the node."}],"type":"exception","reason":"Security must be explicitly enabled when using a [basic] license. Enable security by setting [xpack.security.enabled] to [true] in the elasticsearch.yml file and restart the node."},"status":500}
However, if you send a request to / and receives a response like the following one:
{"error":{"root_cause":[{"type":"security_exception","reason":"missing authentication credentials for REST request [/]","header":{"WWW-Authenticate":"Basic realm=\"security\" charset=\"UTF-8\""}}],"type":"security_exception","reason":"missing authentication credentials for REST request [/]","header":{"WWW-Authenticate":"Basic realm=\"security\" charset=\"UTF-8\""}},"status":401}
That will means that authentication is configured an you need valid credentials to obtain any info from elasticserach. Then, you can try to bruteforce it (it uses HTTP basic auth, so anything that BF HTTP basic auth can be used). Here you have a list default usernames: elastic (superuser), remote_monitoring_user, beats_system, logstash_system, kibana, kibana_system, apm_system, _anonymous_._ Older versions of Elasticsearch have the default password changeme for this user
curl -X GET http://user:password@IP:9200/