In Elasticsearch versions 6.0.0-beta1 to 6.2.4 a disclosure flaw was found in the _snapshot API. When the access_key and security_key parameters are set using the _snapshot API they can be exposed as plain text by users able to query the _snapshot API.
https://www.elastic.co/community/security
https://discuss.elastic.co/t/elastic-stack-6-3-0-and-5-6-10-security-update/135777