Enable MongoDB Access Control

The default data directory for MongoDB is /data/db

This can be overridden by a dbpath option specified on the command line or in a configuration file.

If you install MongoDB via a package manager such as Homebrew or MacPorts these installs typically create a default data directory other than /data/db and set the dbpath in a configuration file.

You can check the dbpath by:

db.serverCmdLineOpts()

in your mongo shell

 "storage" : {
    "dbPath" : "/usr/local/var/mongodb"
 }, 

The following procedure first adds a user administrator to a MongoDB instance running without access control and then enables access control.

1.  Start MongoDB without access control.

$ mongod --port 27017 --dbpath /data/db1

2. Connect to the instance.

$ mongo --port 27017

3. Create the user administrator.

In the admin database, add a user with the userAdminAnyDatabase role. For example, the following creates the user myUserAdmin in the admin database:

Note: The database where you create the user (in this example, admin) is the user’s authentication database.

 > db.createUser(
 ...   {
 ...     user: "abhilash",
 ...     pwd: “password!“,
 ...     roles: [ { role: "userAdminAnyDatabase", db: "admin" } ]
 ...   }
 ... ) 

 Successfully added user: {
 "user" : "abhilash",
 "roles" : [
  {
    "role" : "userAdminAnyDatabase",
    "db" : "admin"
  }
 ]
 } 

Disconnect the mongo shell.

4. Re-start the MongoDB instance with access control.

$ mongod --auth --port 27017 --dbpath /usr/local/var/mongodb/db1

Clients that connect to this instance must now authenticate themselves as a MongoDB user.

 > db.auth();
 Error: auth expects either (username, password) or ({ user: username, pwd: password })
 0
 > db
 test 

** To authenticate during connection:

$ mongo --port 27017 -u "abhilash" -p "password!” --authenticationDatabase "admin"

** To authenticate after connecting

Connect the mongo shell

 $ mongo
 > use admin
 > db.auth("abhilash", “password!“ )
 > mongo
 MongoDB shell version v3.4.7
 connecting to: mongodb://127.0.0.1:27017
 MongoDB server version: 3.4.7
 Server has startup warnings:
 2018-01-22T10:32:18.027+0530 I CONTROL  [initandlisten]
 2018-01-22T10:32:18.027+0530 I CONTROL  [initandlisten] ** WARNING: Access control is not enabled for the database.
 2018-01-22T10:32:18.027+0530 I CONTROL  [initandlisten] **          Read and write access to data and configuration is unrestricted.
 2018-01-22T10:32:18.027+0530 I CONTROL  [initandlisten]
 > use admin
 switched to db admin
 > db
 admin
 > db.auth("abhilash", "password!”);
 1
 > use my_dbname;
 > db.createUser(
   {
     user: "vadmin",
     pwd: “pass111!”,
     roles: [ { role: "readWrite", db: "my_dbname" },
              { role: "read", db: "test" } ]
   }
 ) 

Reference:  Mongodb enable-authentication