How to Integrate Multi-Factor Authentication in AWS Cognito with Custom Lambdas

So, you’re looking to beef up your AWS Cognito security with multi-factor authentication (MFA), and you want to do it your way using custom Lambda functions. That’s a smart move. While Cognito offers built-in MFA options, sometimes you need a bit more control, a specific workflow, or integration with a particular service. This guide will walk you through how to achieve that, focusing on practical steps and real-world scenarios.

The “Why” Behind Custom MFA in Cognito

Before we dive into the “how,” let’s quickly touch on why you might choose custom MFA over the out-of-the-box solutions. Cognito’s standard MFA (like SMS or TOTP) is great for many use cases. However, you might need custom MFA if:

• You have unique authentication factors: Perhaps you need to integrate with a hardware security key, a custom biometrics service, or a proprietary internal system for a second factor.
• You require specific user flows: Maybe you need to enforce MFA only for certain user groups, trigger MFA based on specific application events, or have a more complex step-up authentication process.
• You need advanced logging and auditing: Custom Lambda functions give you fine-grained control over what data is logged during the MFA process, which can be crucial for compliance or detailed security analysis.
• You want to integrate with existing infrastructure: If you already have an MFA solution in place that you want to leverage, a custom Lambda can act as the bridge.

Essentially, custom MFA offers flexibility. It lets you build MFA that precisely fits your application’s security posture and user experience.

To get started with custom MFA in Cognito using Lambda, you’ll need a few things in place. It’s not overly complicated, but having these ready will smooth the process.

Essential AWS Services

• AWS Cognito User Pool: This is the core service for managing your users. You’ll need to create one if you haven’t already.
• AWS Lambda: This is where your custom MFA logic will live. You’ll write and deploy your code here.
• Amazon API Gateway (Optional but recommended): While not strictly required for every custom MFA flow, API Gateway is often used to expose your Lambda functions as callable endpoints. This is particularly useful if your MFA logic needs to be invoked by your client application or another service.
• AWS IAM (Identity and Access Management): You’ll need to set up roles and permissions for your Lambda functions to interact with other AWS services.

Cognito User Pools have several “triggers” that allow you to hook into different stages of the user authentication lifecycle. For MFA, the primary trigger you’ll be interested in is the Custom Message trigger and, more importantly, the Define Auth Challenge and Create Auth Challenge triggers.

• Define Auth Challenge: This trigger is invoked before Cognito decides whether to issue an MFA challenge or move to the next step. It’s where you’ll tell Cognito whether MFA is needed.
• Create Auth Challenge: If Define Auth Challenge signals that an MFA challenge is required, this trigger is invoked. This is where you’ll create the actual challenge that’s presented to the user.
• Custom Message: This trigger can be used to send custom messages to users, which could include the code for their MFA.
• Verify Auth Challenge Response: This trigger is invoked when the user provides their response to a challenge (e.g., entering an MFA code). You’ll use this to validate their response.

Key Takeaways

Building Your Custom MFA Logic: A Step-by-Step Approach

Let’s break down the process of creating your custom MFA. We’ll focus on a common scenario: using a custom MFA provider (like sending a code via a custom SMS service or a proprietary app).

Step 1: Configure Your Cognito User Pool for Custom MFA

First, you need to tell your Cognito User Pool that you intend to use custom authentication challenges.

Enabling Custom Authentication Flows

Navigate to your Cognito User Pool in the AWS Management Console. Under “App integration,” you’ll find “Authentication flows.” Here, you’ll need to enable “Custom authentication flows.” This is the crucial step that allows Cognito to invoke your Lambda triggers for authentication challenges.

Selecting Trigger Functions

Within your User Pool settings, find the “Triggers” section. You’ll see various triggers. For custom MFA, you’ll want to configure:

• Define Auth Challenge: This is where you’ll link your Lambda function that determines if MFA is needed.
• Create Auth Challenge: This is where you’ll link your Lambda function that generates the MFA challenge.
• Verify Auth Challenge Response: This is where you’ll link your Lambda function that validates the user’s MFA response.
• Custom Message (Optional): If your MFA mechanism involves sending a code, this trigger can be used to customize the message containing that code.

For each of these, you’ll select “Lambda function” and then choose the specific Lambda function you’ll create or have already created.

Step 2: The Define Auth Challenge Lambda Function

This Lambda function is the gatekeeper. It decides whether an MFA challenge should be presented to the user.

What the Lambda Does

The Define Auth Challenge trigger receives an event object from Cognito. This event contains information about the user, the current authentication state, and any previous challenges. Your Lambda’s job is to inspect this data and decide what happens next.

Key Event Properties to Consider

• request.session: An array of challenges that have already been presented. If this array is empty, it’s the first challenge.
• request.userNotFound: Indicates if the user was not found.
• request.userAttributes: User attributes from Cognito.
• challengeName: The name of the challenge that Cognito is considering.

The Lambda’s Output

Your Lambda function should return an object with:

• session: This array needs to be populated with the names of the challenges Cognito should execute. If you want to trigger your custom MFA, you’ll add its name here.
• issueTokens: A boolean indicating whether to issue tokens. Typically, you set this to true if all challenges are successfully met.
• failAuthentication: A boolean indicating whether to fail authentication.

Example Logic (Conceptual)

“`javascript

exports.handler = async (event) => {

// Check if this is the first time we’re encountering a challenge

if (event.request.session.length === 0) {

// If it’s the first challenge, add our custom MFA challenge

event.response.session.push(‘CUSTOM_AUTH_CHALLENGE’); // Use a unique name for your challenge

} else {

// If there have been previous challenges, and our custom one was met,

// we might not need to add more. Or if we want sequential MFA.

// For simplicity here, let’s assume we only need one custom challenge.

// If the previous challenge was our custom one and it succeeded,

// we don’t add it again.

const lastChallenge = event.request.session[event.request.session.length – 1];

if (lastChallenge !== ‘CUSTOM_AUTH_CHALLENGE’) {

event.response.session.push(‘CUSTOM_AUTH_CHALLENGE’);

}

}

// If the user was not found, we might want to fail immediately

if (event.request.userNotFound) {

event.response.failAuthentication = true;

} else {

// If we’ve added our custom challenge, we don’t issue tokens yet.

// Cognito will handle token issuance after all challenges are met.

event.response.issueTokens = false;

event.response.failAuthentication = false;

}

return event;

};

“`

Step 3: The Create Auth Challenge Lambda Function

This is where the magic happens to present the actual MFA challenge to the user.

What the Lambda Does

The Create Auth Challenge trigger is invoked when Define Auth Challenge has added a challenge to the session array. This Lambda’s responsibility is to prepare the challenge object that will be sent back to the client.

Key Event Properties to Consider

• request.challengeName: The name of the challenge to create (e.g., CUSTOM_AUTH_CHALLENGE).
• request.userAttributes: User attributes, which you might need to get an MFA device identifier or phone number.
• session: The current session of challenges.

The Lambda’s Output

Your Lambda function should return an object with:

• challengeMetadata: A string to pass between challenge functions. This is useful for storing temporary state.
• publicChallengeParameters: Parameters that are sent back to the client. This could include instructions for the user or a challenge ID.
• privateChallengeParameters: Parameters that are not sent back to the client. These are for your backend logic only.
• challengeName: The name of the challenge.

Example Logic (Conceptual for custom SMS code)

Let’s imagine you’re sending a custom SMS code to a user’s pre-registered phone number.

“`javascript

const AWS = require(‘aws-sdk’);

const twilioClient = require(‘twilio’)(‘YOUR_TWILIO_ACCOUNT_SID’, ‘YOUR_TWILIO_AUTH_TOKEN’); // Example for Twilio

exports.handler = async (event) => {

const challengeName = event.request.challengeName;

const phoneNumber = event.request.userAttributes.phone_number; // Assuming phone_number is a user attribute

if (challengeName === ‘CUSTOM_AUTH_CHALLENGE’) {

// Generate a random 6-digit code

const code = Math.floor(100000 + Math.random() * 900000).toString();

// Store the code in the user’s session or a temporary store (e.g., DynamoDB)

// For simplicity, we’ll store it in privateChallengeParameters for this example.

// Important Security Note: Storing sensitive data directly in privateChallengeParameters

// is acceptable for short-lived challenges but consider a more robust solution

// like DynamoDB for longer-lived or critical data.

event.response.privateChallengeParameters = {

code: code

};

// Send the code to the user via your custom mechanism (e.g., Twilio SMS)

try {

await twilioClient.messages.create({

to: phoneNumber,

from: ‘YOUR_TWILIO_PHONE_NUMBER’, // Your Twilio verified phone number

body: Your authentication code is: ${code},

});

console.log(Sent MFA code to ${phoneNumber});

} catch (error) {

console.error(‘Error sending SMS:’, error);

throw new Error(‘Failed to send MFA code.’);

}

// Parameters to send back to the client

event.response.publicChallengeParameters = {

prompt: ‘Enter the code sent to your phone.’,

// You might not want to send the actual code here, but maybe a hint

// or a challenge ID that your client can use to poll or display.

};

}

return event;

};

“`

Step 4: The Verify Auth Challenge Response Lambda Function

This is the final step in the MFA flow. It verifies the user’s input.

What the Lambda Does

The Verify Auth Challenge Response trigger receives the user’s response to the challenge, along with any parameters that were stored in privateChallengeParameters during the Create Auth Challenge step.

Key Event Properties to Consider

• request.challengeAnswer: The answer provided by the user.
• request.privateChallengeParameters: Parameters that were stored by the Create Auth Challenge function.
• request.userAttributes: User attributes.

The Lambda’s Output

Your Lambda function should return an object with:

• answerCorrect: A boolean indicating whether the user’s answer was correct.

Example Logic (Continuing the SMS code example)

“`javascript

exports.handler = async (event) => {

const answerCorrect = event.request.challengeAnswer === event.request.privateChallengeParameters.code;

event.response.answerCorrect = answerCorrect;

return event;

};

“`

Step 5: Handling Custom Message Triggers (Optional but Recommended)

If you’re sending codes, you’ll likely want to customize the message. The Custom Message trigger is perfect for this.

What the Lambda Does

This trigger is invoked when Cognito needs to send a message to the user, such as a verification code during sign-up or, in our case, when sending an MFA code.

Key Event Properties to Consider

• request.codeParameter: The code that Cognito generated (if applicable).
• request.userAttributes: User attributes.
• request.deliveryMedium: How the message should be delivered (e.g., SMS, EMAIL).
• request.message: The default message.

The Lambda’s Output

Your Lambda function should return an object with:

• message: The custom message to send.
• smsMessage: The custom SMS message to send.
• emailMessage: The custom email message to send.

Example Logic (Customizing the SMS message)

“`javascript

exports.handler = async (event) => {

let message = ”;

if (event.request.codeParameter) {

// If we’re sending a code (e.g., for MFA)

const code = event.request.codeParameter;

const phoneNumber = event.request.userAttributes.phone_number;

// Use your preferred method to send the message here.

// For this example, we’ll just construct the message.

// If you were using the built-in SMS delivery, you’d return the message.

// If you have a separate SMS sending mechanism (like in Create Auth Challenge),

// you might just use this trigger to get the code and format the message.

message = Your security code for authentication is: ${code}. Please enter it in the app.;

console.log(Customizing message for phone number ${phoneNumber} with code ${code});

} else {

// Fallback for other types of messages if needed

message = event.message;

}

event.response.smsMessage = message;

event.response.emailMessage = message; // Can customize email separately if needed

event.response.message = message;

Note: If your Create Auth Challenge Lambda directly sends the SMS using a third-party service like Twilio, you might not strictly need the Custom Message trigger for the MFA code itself. However, it’s still useful for other Cognito-initiated messages.

Integrating with Your Client Application

The Lambda functions handle the backend logic, but your client application needs to interact with these challenges.