[mongodb] MongoDB what are the default user and password?

I am using the same connection string on local and production. When the connection string is mongodb://localhost/mydb

What is the username and password? Is it secure to keep it this way?

For MongoDB earlier than 2.6, the command to add a root user is addUser (e.g.)

db.addUser({user:'admin',pwd:'<password>',roles:["root"]})

In addition to previously provided answers, one option is to follow the 'localhost exception' approach to create the first user if your db is already started with access control (--auth switch). In order to do that, you need to have localhost access to the server and then run:

mongo
use admin
db.createUser(
 {
     user: "user_name",
     pwd: "user_pass",
     roles: [
           { role: "userAdminAnyDatabase", db: "admin" },
           { role: "readWriteAnyDatabase", db: "admin" },
           { role: "dbAdminAnyDatabase", db: "admin" }
        ]
 })

As stated in MongoDB documentation:

The localhost exception allows you to enable access control and then create the first user in the system. With the localhost exception, after you enable access control, connect to the localhost interface and create the first user in the admin database. The first user must have privileges to create other users, such as a user with the userAdmin or userAdminAnyDatabase role. Connections using the localhost exception only have access to create the first user on the admin database.

Here is the link to that section of the docs.

In addition with what @Camilo Silva already mentioned, if you want to give free access to create databases, read, write databases, etc, but you don't want to create a root role, you can change the 3rd step with the following:

use admin
db.createUser(
  {
    user: "myUserAdmin",
    pwd: "abc123",
    roles: [ { role: "userAdminAnyDatabase", db: "admin" }, 
             { role: "dbAdminAnyDatabase", db: "admin" }, 
             { role: "readWriteAnyDatabase", db: "admin" } ]
  }
)