Elasticsearch Security versions 6.4.0 to 6.4.2 contain an error in the way request headers are applied to requests when using the Active Directory, LDAP, Native, or File realms. A request may receive headers intended for another request if the same username is being authenticated concurrently; when used with run as, this can result in the request running as the incorrect user. This could allow a user to access information that they should not have access to.
https://www.elastic.co/community/security
https://discuss.elastic.co/t/elastic-stack-6-4-3-and-5-6-13-security-update/155594