MongoDB Authentication Issues with Node.js: Fixing Connection Problems with authSource

🎫 Content is validated at 2025-08-15 for

✅ Node.js 18+

✅ MongoDB 5+

✅ Mongoose 7+

Background: MongoDB Authentication and Connection Issues

When connecting to MongoDB using Mongoose in Node.js applications, developers often encounter authentication errors like:

errmsg: 'Authentication failed.',
code: 18,
codeName: 'AuthenticationFailed'

These errors can be particularly confusing when the username, password, and database name are all correct. One common cause of this issue is not specifying the correct authentication database, which can be resolved by adding authSource: 'admin' to your connection options.

Understanding MongoDB Authentication Database

In MongoDB, authentication is handled by the admin database by default. When you create users in MongoDB, they can be associated with specific databases, but the authentication process itself typically occurs through the admin database.

When you don't specify the authSource parameter in your connection string or options, MongoDB might try to authenticate against the target database instead of the admin database where the user credentials are actually stored.

The Problem: Authentication Failure

Let's look at a common incorrect connection pattern:

const mongoose = require('mongoose');

// ❌ Incorrect - Missing authSource
const connectionString = 'mongodb://username:password@localhost:27017/myDatabase';
mongoose.connect(connectionString);

// ❌ Also incorrect - Still missing authSource
mongoose.connect('mongodb://localhost:27017/myDatabase', {
  auth: {
    username: 'myUser',
    password: 'myPassword'
  }
});

Both of these connection attempts might fail with authentication errors, even if your username and password are correct.

The Solution: Adding authSource

Here are the correct ways to connect to MongoDB with the proper authentication source:

Method 1: Using Connection String with authSource

const mongoose = require('mongoose');

// ✅ Correct - Adding authSource to the connection string
const connectionString = 'mongodb://username:password@localhost:27017/myDatabase?authSource=admin';
mongoose.connect(connectionString);

Method 2: Using Connection Options

const mongoose = require('mongoose');

// ✅ Correct - Adding authSource in connection options
mongoose.connect('mongodb://localhost:27017/myDatabase', {
  auth: {
    username: 'myUser',
    password: 'myPassword'
  },
  authSource: 'admin'  // This is the key addition
});

Environment Variables Setup

It's a best practice to store your connection details in environment variables:

.env file:

MONGODB_URI=mongodb://localhost:27017/myDatabase
DB_USER=myUsername
DB_PASS=myPassword

Connection code:

const mongoose = require('mongoose');

const connectDB = async () => {
  const uri = `${process.env.MONGODB_URI}?authSource=admin`;

  try {
    const conn = await mongoose.connect(uri, {
      auth: {
        username: process.env.DB_USER,
        password: process.env.DB_PASS
      },
      authSource: 'admin'
    });

    console.log(`MongoDB Connected: ${conn.connection.host}`);
  } catch (error) {
    console.error('MongoDB connection error:', error);
    process.exit(1);
  }
};

connectDB();

Common Scenarios Where authSource is Required

1. MongoDB Atlas: When using MongoDB Atlas, you typically need to specify authSource=admin
2. Dockerized MongoDB: Containers running MongoDB with authentication enabled
3. Production MongoDB instances: Most production MongoDB setups require explicit authentication source
4. Custom MongoDB deployments: When authentication database is not the same as the target database

Alternative authSource Values

While admin is the most common value for authSource, you might encounter other scenarios:

// When user is created in the same database
mongoose.connect('mongodb://localhost:27017/myDatabase', {
  auth: {
    username: 'myUser',
    password: 'myPassword'
  },
  authSource: 'myDatabase'  // User created in this database
});

// When using a custom authentication database
mongoose.connect('mongodb://localhost:27017/myDatabase', {
  auth: {
    username: 'myUser',
    password: 'myPassword'
  },
  authSource: 'authDB'  // Custom authentication database
});

Troubleshooting Tips

1. Check User Permissions: Ensure your MongoDB user has appropriate roles:

   // In MongoDB shell
   db.createUser({
     user: "myUser",
     pwd: "myPassword",
     roles: [
       { role: "readWrite", db: "myDatabase" },
       { role: "dbAdmin", db: "myDatabase" }
     ]
   });

2. Verify Connection String: Double-check all parts of your connection string:

   mongodb://[username:password@]host:port/database?authSource=admin

3. Check MongoDB Logs: Look at MongoDB server logs for more detailed error messages.

4. Test with MongoDB Shell: Try connecting with the mongo shell first:

   mongo mongodb://username:password@localhost:27017/myDatabase?authSource=admin

Summary

The authSource: 'admin' parameter is crucial for successful MongoDB authentication in many scenarios. When you encounter authentication errors with Mongoose, always check if you're properly specifying the authentication source. This simple addition can resolve many connection issues and is a common requirement for production MongoDB deployments.

Remember to:

• Always specify authSource when connecting to authenticated MongoDB instances
• Use environment variables for sensitive connection information
• Handle connection errors gracefully
• Test your connection in development before deploying to production