Link: http://mms.mongodb.com/help/reference/authentication/

This page documents the required roles to monitor MongoDB 2.6 and 2.4 instances separately. However, it doesn't document the required roles to backup MongoDB 2.6 instances. Currently the document on the required roles for the backup agent and the command to add the user only applies to MongoDB 2.4 instances.

In MongoDB 2.6, there is a new built-in role backup that can be used for the backup agent. The command to add a user with backup role in MongoDB 2.6 is:

use admin
db.createUser( { user: "<username>",
                 pwd: "<password>",
                 roles: [
                   { role: "backup", db: "admin" }
                 ] } )