X-Pack security has three internal users (_system, _xpack, and _xpack_security) that are responsible for the operations that take place inside an Elasticsearch cluster.

These users are only used by requests that originate from within the cluster. For this reason, they cannot be used to authenticate against the API and there is no password to manage or reset.

From time-to-time you may find a reference to one of these users inside your logs, including audit logs.