What is the default user and password for elasticsearch?

Defaults are:

user: elastic
password: changeme

So:

$ curl -u elastic:changeme localhost:9200
{
  "name" : "5aEHJ-Y",
  "cluster_name" : "docker-cluster",
  "cluster_uuid" : "3FmaYN7rS56oBTqWOyxmKA",
  "version" : {
    "number" : "5.6.2",
    "build_hash" : "57e20f3",
    "build_date" : "2017-09-23T13:16:45.703Z",
    "build_snapshot" : false,
    "lucene_version" : "6.6.1"
  },
  "tagline" : "You Know, for Search"
}

Read more about changing the defaults.