The existing Elasticsearch sink does not support Elasticsearch clusters that have been secured with Shield. (See https://www.elastic.co/products/shield) All that would be required to add this support is allowing users to configure the username and password on the sink in their flume.conf and then setting the username and password on the HTTP header like so:
UsernamePasswordCredentials creds = new UsernamePasswordCredentials("username", "password");
HttpPost httpRequest = new HttpPost(url);
httpRequest.addHeader(new BasicScheme().authenticate(creds, httpRequest));
I've already hard-coded a quick fix for myself that works, but it would be awesome to see this in a future release and/or patch.