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S3 OAuth2 Authentication with Okta

This module provides OAuth2 authentication with Okta for an existing S3 bucket configured as static website.

The resources created by this module will be deployed in us-east-1 because Lambda@Edge can only be deployed in this region (the Lambda is then replicated all over the world by AWS).

The Lambda is written in NodeJS because Lambda@Edge requires that the Lambda package size, including dependencies, is at most 1 MB. Other runtimes such as Python don't include as much features as NodeJS in their standard library (at least for the specific features we need) and so the Lambda package would be too large with these runtimes.

Be careful, this module will put in place a S3 bucket policy for the existing S3 bucket, if your bucket already have one it will be overwritten.

Deployment

A Terraform deployment example is provided in the ./aws-s3-oauth2-okta/example-deployment folder.

1. Create the Okta application

Create the application with the correct redirect URI and retrieve the client ID and client secret.

2. Install NPM packages

Go to ./aws-s3-oauth2-okta/module/okta_auth_lambda_package and run npm install --only=prod.

3. Apply!

We assume here that you have an AWS environment configured locally.

Go back to ./aws-s3-oauth2-okta/example-deployment and run:

  1. Initialize Terraform: terraform init
  2. Apply:
TF_VAR_aws_account_id=${AWS_ACCOUNT_ID} \
TF_VAR_okta_client_id=${OKTA_CLIENT_ID} \
TF_VAR_okta_client_secret=${OKTA_CLIENT_SECRET} \
TF_VAR_okta_domain=${OKTA_DOMAIN} \
terraform apply

How it works

The main building blocks of the solution are:

  • A CloudFront distribution with a CloudFront Origin Access Identity (OAI)
  • A Lambda@Edge

The bucket access is configured as private and so the only way to access it is via the CloudFront OAI which is allowed to read it thanks to an S3 bucket policy.

The CloudFront distribution has a Lambda@Edge configured on the viewer-request event (see image below). It means that every time a request is made, it first goes through the Lambda before reaching the Origin (the S3 bucket).

The Lambda will then decide what to do based on the authentication Cookie that the user submitted (or not) in the request. It will either:

  • Redirect the user to Okta if there is no Cookie or if it is invalid/expired
  • Display a 401 Unauthorized response if Okta returned an invalid Auth Code
  • Redirect the user to /index.html if the login was successful (i.e. Okta returned a valid Auth Code)
  • Allow the request to continue to the Origin if the authentication Cookie is valid

Note: the authentication cookie actually contains a signed JWT, that's how the Lambda knows it is legit.

CloudFront events that trigger Lambda functions