Securing Lambda Function URLs using Amazon Cognito, Amazon CloudFront and AWS WAF

This post is written by Madhu Singh (Solutions Architect), and Krupanidhi Jay (Solutions Architect).

Lambda function URLs is a dedicated HTTPs endpoint for a AWS Lambda function. You can configure a function URL to have two methods of authentication: IAM and NONE. IAM authentication means that you are restricting access to the function URL (and in-turn access to invoke the Lambda function) to certain AWS principals (such as roles or users). Authentication type of NONE means that the Lambda function URL has no authentication and is open for anyone to invoke the function.

This blog shows how to use Lambda function URLs with an authentication type of NONE and use custom authorization logic as part of the function code, and to only allow requests that present valid Amazon Cognito credentials when invoking the function. You also learn ways to protect Lambda function URL against common security threats like DDoS using AWS WAF and Amazon CloudFront.

Lambda function URLs provides a simpler way to invoke your function using HTTP calls. However, it is not a replacement for Amazon API Gateway, which provides advanced features like request validation and rate throttling.

Solution overview

There are four core components in the example.

1. A Lambda function with function URLs enabled

At the core of the example is a Lambda function with the function URLs feature enabled with the authentication type of NONE. This function responds with a success message if a valid authorization code is passed during invocation. If not, it responds with a failure message.

2. Amazon Cognito User Pool

Amazon Cognito user pools enable user authentication on websites and mobile apps. You can also enable publicly accessible Login and Sign-Up pages in your applications using Amazon Cognito user pools’ feature called the hosted UI.

In this example, you use a user pool and the associated Hosted UI to enable user login and sign-up on the website used as entry point. This Lambda function validates the authorization code against this Amazon Cognito user pool.

3. CloudFront distribution using AWS WAF

CloudFront is a content delivery network (CDN) service that helps deliver content to end users with low latency, while also improving the security posture for your applications.

AWS WAF is a web application firewall that helps protect your web applications or APIs against common web exploits and bots and AWS Shield is a managed distributed denial of service (DDoS) protection service that safeguards applications running on AWS. AWS WAF inspects the incoming request according to the configured Web Access Control List (web ACL) rules.

Adding CloudFront in front of your Lambda function URL helps to cache content closer to the viewer, and activating AWS WAF and AWS Shield helps in increasing security posture against multiple types of attacks, including network and application layer DDoS attacks.

4. Public website that invokes the Lambda function

The example also creates a public website built on React JS and hosted in AWS Amplify as the entry point for the demo. This website works both in authenticated mode and in guest mode. For authentication, the website uses Amazon Cognito user pools hosted UI.

Solution architecture

This shows the architecture of the example and the information flow for user requests.

In the request flow:

The entry point is the website hosted in AWS Amplify. In the home page, when you choose “sign in”, you are redirected to the Amazon Cognito hosted UI for the user pool.
Upon successful login, Amazon Cognito returns the authorization code, which is stored as a cookie with the name “code”. The user is redirected back to the website, which has an “execute Lambda” button.
When the user choose “execute Lambda”, the value from the “code” cookie is passed in the request body to the CloudFront distribution endpoint.
The AWS WAF web ACL rules are configured to determine whether the request is originating from the US or Canada IP addresses and to determine if the request should be allowed to invoke Lambda function URL origin.
Allowed requests are forwarded to the CloudFront distribution endpoint.
CloudFront is configured to allow CORS headers and has the origin set to the Lambda function URL. The request that CloudFront receives is passed to the function URL.
This invokes the Lambda function associated with the function URL, which validates the token.
The function code does the following in order:
1. Exchange the authorization code in the request body (passed as the event object to Lambda function) to access_token using Amazon Cognito’s token endpoint (check the documentation for more details).
  1. Amazon Cognito user pool’s attributes like user pool URL, Client ID and Secret are retrieved from AWS Systems Manager Parameter Store (SSM Parameters).
  2. These values are stored in SSM Parameter Store at the time these resources are deployed via AWS CDK (see “how to deploy” section)
2. The access token is then verified to determine its authenticity.
3. If valid, the Lambda function returns a message stating user is authenticated as <username> and execution was successful.
4. If either the authorization code was not present, for example, the user was in “guest mode” on the website, or the code is invalid or expired, the Lambda function returns a message stating that the user is not authorized to execute the function.
The webpage displays the Lambda function return message as an alert.

Getting started

Pre-requisites:

Before deploying the solution, please follow the README from the GitHub repository and take the necessary steps to fulfill the pre-requisites.

Deploy the sample solution

1. From the code directory, download the dependencies:

$ npm install

2. Start the deployment of the AWS resources required for the solution:

$ cdk deploy

Note:

optionally pass in the –profile argument if needed
The deployment can take up to 15 minutes

3. Once the deployment completes, the output looks similar to this:

Open the amplifyAppUrl from the output in your browser. This is the URL for the demo website. If you don’t see the “Welcome to Compute Blog” page, the Amplify app is still building, and the website is not available yet. Retry in a few minutes. This website works either in an authenticated or unauthenticated state.

Test the authenticated flow

To test the authenticated flow, choose “Sign In”.

2. In the sign-in page, choose on sign-up (for the first time) and create a user name and password.

3. To use an existing an user name and password, enter those credentials and choose login.

4. Upon successful sign-in or sign up, you are redirected back to the webpage with “Execute Lambda” button.

5. Choose this button. In a few seconds, an alert pop-up shows the logged in user and that the Lambda execution is successful.

Testing the unauthenticated flow

1. To test the unauthenticated flow, from the Home page, choose “Continue”.

2. Choose “Execute Lambda” and in a few seconds, you see a message that you are not authorized to execute the Lambda function.

Testing the geo-block feature of AWS WAF

1. Access the website from a Region other than US or Canada. If you are physically in the US or Canada, you may use a VPN service to connect to a Region other than US or Canada.

2. Choose the “Execute Lambda” button. In the Network trace of browser, you can see the call to invoke Lambda function was blocked with Forbidden response.

3. To try either the authenticated or unauthenticated flow again, choose “Return to Home Page” to go back to the home page with “Sign In” and “Continue” buttons.

Cleaning up

To delete the resources provisioned, run the cdk destroy command from the AWS CDK CLI.

Conclusion

In this blog, you create a Lambda function with function URLs enabled with NONE as the authentication type. You then implemented a custom authentication mechanism as part of your Lambda function code. You also increased the security of your Lambda function URL by setting it as Origin for the CloudFront distribution and using AWS WAF Geo and IP limiting rules for protection against common web threats, like DDoS.

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