What does the error “not authorized on [database] to execute command” mean, and how can you resolve it?

 The MongoDB error “not authorized on [database] to execute command” means that the authenticated user does not have the necessary permissions (roles) to run the specified command on the given database.

🔍 Why This Happens

MongoDB uses a Role-Based Access Control (RBAC) system. Each user is assigned roles that grant specific privileges on databases. This error typically occurs when:

• The user is trying to read, write, or perform an administrative task on a database they don't have access to.

• A script or application is attempting an operation (e.g., insert, createIndex, aggregate) without the required privilege.

• The user is authenticated against the wrong database (especially common with external auth).

🛠️ How to Fix It

✅ 1. Check the User's Roles

Log in with an admin user and check the current user’s roles:

use admin
db.getUser("yourUsername")

This will list all roles and databases the user has access to.

✅ 2. Grant the Appropriate Role

To grant access, use:

use <database>
db.grantRolesToUser("yourUsername", [
  { role: "readWrite", db: "<database>" }
])

Common built-in roles include:

• read: Read-only access to a database

• readWrite: Read/write access to a database

• dbAdmin: Administrative tasks (like creating indexes)

• userAdmin: Manage users and roles

• clusterAdmin: Cluster-level operations

✅ 3. Authenticate on the Correct Database

Users are typically created in a specific database. If you're authenticating on the wrong one, access might fail:

Example (Mongo shell):

mongo -u yourUsername -p yourPassword --authenticationDatabase yourUserDB

✅ 4. Check Your Application Connection String

Ensure the connection URI specifies the correct authentication database:

mongodb://yourUsername:yourPassword@host:port/yourAuthDB

Or add:

?authSource=yourAuthDB

Example:

mongodb://user:pass@localhost:27017/mydb?authSource=admin

✅ 5. Use a Role That Matches the Command

If you're trying to do something like create a collection, run an aggregation with $out, or modify indexes, make sure the user has dbAdmin or more advanced roles like dbOwner.

✅ Example Fix Scenario

Error:

not authorized on mydb to execute command { insert: "users", ... }

Fix:

use mydb
db.grantRolesToUser("myuser", [{ role: "readWrite", db: "mydb" }])

Summary

• The error means the user lacks permission for the attempted action.

• Fix it by granting appropriate roles or correcting the authentication database.