WARNING: Access control is not enabled for the database.

# mongo
MongoDB shell version v4.2.6
connecting to: mongodb://127.0.0.1:27017/?compressors=disabled&gssapiServiceName=mongodb
Implicit session: session { "id" : UUID("ca2e8d54-ee4c-4db7-becb-163bdbbea52d") }
MongoDB server version: 4.2.6
Server has startup warnings:
2020-04-29T03:45:58.052+0700 I  CONTROL  [initandlisten]
2020-04-29T03:45:58.053+0700 I  CONTROL  [initandlisten] ** WARNING: Access control is not enabled for the database.
2020-04-29T03:45:58.053+0700 I  CONTROL  [initandlisten] **          Read and write access to data and configuration is unrestricted.
2020-04-29T03:45:58.053+0700 I  CONTROL  [initandlisten]
---

Set up Admin in MongoDB

# mongo

use admin
db.createUser(
  {
    user: "mongo_root",
    pwd: passwordPrompt(), // or cleartext password
    roles: [ { role: "userAdminAnyDatabase", db: "admin" }, "readWriteAnyDatabase" ]
  }
)

# vim /etc/mongod.conf

security:
    authorization: enabled

# systemctl restart mongod

To access your database after you set admin and password

# mongo --authenticationDatabase "admin" -u "mongo_root" -p

So the warning message will be gone.

# mongo --authenticationDatabase "admin" -u "mongo_root" -p
MongoDB shell version v4.2.6
Enter password:
connecting to: mongodb://127.0.0.1:27017/?authSource=admin&compressors=disabled&gssapiServiceName=mongodb
Implicit session: session { "id" : UUID("83818706-d522-4c1b-a5bf-6ef2fa8c80d1") }
MongoDB server version: 4.2.6