OAuth 2.0 Bearer JWT Authorizer for AWS API Gateway

This project is sample implementation of an AWS Lambda custom authorizer for AWS API Gateway that works with a JWT bearer token (id_token or access_token) issued by an OAuth 2.0 Authorization Server. It can be used to secure access to APIs managed by AWS API Gateway.

Configuration

Environment Variables (.env)

Update the ISSUER and AUDIENCE variables in the .env file

ISSUER=https://example.oktapreview.com/oauth2/aus8o56xh1qncrlwT0h7
AUDIENCE=https://api.example.com

It is critical that the issuer and audience claims for JWT bearer tokens are properly validated using best practices. You can obtain these values from your OAuth 2.0 Authorization Server configuration.

The audience value should uniquely identify your AWS API Gateway deployment. You should assign unique audiences for each API Gateway authorizer instance so that a token intended for one gateway is not valid for another.

Signature Keys (keys.json)

Update keys.json with the JSON Web Key Set (JWKS) format for your issuer. You can usually obtain the JWKS for your issuer by fetching the jwks_uri published in your issuer's metadata such as ${issuer}/.well-known/openid-configuration.

The authorizer only supports RSA signature keys

Ensure that your issuer uses a pinned key for token signatures and does not automatically rotate signing keys. The authorizer currently does not support persistence of cached keys (e.g. dynamo) obtained via metadata discovery.

Scopes

This sample currently enforces scope-based access to API resources using the scp claim in the JWT. The api:read scope is required for GET requests and api:write scope for POST, PUT, PATCH, or DELETE requests.

Update index.js with your authorization requirements and return the resulting AWS IAM Policy for the request.

Deployment

Install Dependencies

Run npm install to download all of the authorizer's dependent modules. This is a prerequisite for deployment as AWS Lambda requires these files to be included in the uploaded bundle.

Create Bundle

You can create the bundle using npm run zip. This creates a oauth2-jwt-authorizer.zip deployment package in the dist folder with all the source, configuration and node modules AWS Lambda needs.

Create Lambda function

From the AWS Lambda console

• Name: oauth2-jwt-authorizer

• Description: OAuth2 Bearer JWT authorizer for API Gateway

• Runtime: Node.js 4.3

• Code entry type: Upload a .ZIP file

• Upload: select dist\lambda-oauth2-jwt-authorizer.zip we created in the previous step

• Handler: index.handler

• Role: select an existing role with lambda:InvokeFunction action

  If you don't have an existing role, you will need to create a new role as outlined below

• Memory (MB): 128

• Timeout: 30 seconds

• VPC: No VPC

Click Next and Create

Create IAM Role

You will need to create an IAM Role that has permissions to invoke the Lambda function we created above.

That Role will need to have a Policy similar to the following:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Resource": [
                "*"
            ],
            "Action": [
                "lambda:InvokeFunction"
            ]
        }
    ]
}

Configure API Gateway

From the AWS API Gateway console

Open your API, or Create a new one.

In the left panel, under your API name, click on Custom Authorizers. Click on Create

• Name: oauth2-jwt-authorizer

• Lambda region: from previous step

• Execution role: the ARN of the Role we created in the previous step

• Identity token source: method.request.header.Authorization

• Token validation expression: ^Bearer [-0-9a-zA-z\.]*$

  Cut-and-paste this regular expression from ^ to $ inclusive

• Result TTL in seconds: 300

Click Create

Testing

You can test the authorizer by supplying an id_token or access_token and clicking Test

Bearer <token>

A successful test will look something like:

Latency: 1000 ms
Principal Id: user@example.com
Policy
{
    "Version": "2012-10-17",
    "Statement": [
        {
        "Sid": "Stmt1459758003000",
        "Effect": "Allow",
        "Action": [
            "execute-api:Invoke"
        ],
        "Resource": [
            "arn:aws:execute-api:*"
        ]
        }
    ]
}

Configure API Gateway Methods to use the Authorizer

In the left panel, under your API name, click on Resources. Under the Resource tree, select one of your Methods (POST, GET etc.)

Select Method Request. Under Authorization Settings change:

• Authorizer : oauth2-jwt-authorizer

Make sure that:

• API Key Required : false

Click the tick to save the changes.

Deploy the API

You need to Deploy the API to make the changes public.

Select Action and Deploy API. Select your Stage.

Test your endpoint remotely

With Postman

You can use Postman to test the REST API

• Method: < matching the Method in API Gateway >
• URL https://<api-id>.execute-api.<region>.amazonaws.com/<stage>/<resource>
• The base URL you can see in the Stages section of the API
• Append the Resource name to get the full URL
• Header - add an Authorization key
• Authorization : Bearer

With curl from the command line

$ curl -X POST <url> -H 'Authorization: Bearer <token>'

In (modern) browsers console with fetch

fetch( '<url>', { method: 'POST', headers: { Authorization : 'Bearer <token>' }}).then(response => { console.log( response );});