Setting Up CORS and Integration on AWS API Gateway Using CloudFormation

Cross-Origin Resource Sharing (CORS) is an essential security mechanism utilized by web browsers, allowing for regulated access to server resources from origins that differ in domain, protocol, or port. In the realm of APIs, especially when utilizing AWS API Gateway, configuring CORS is crucial to facilitate access for web applications originating from various domains while mitigating potential security risks.

This article aims to provide a comprehensive guide on CORS and integrating AWS API Gateway through CloudFormation. It will emphasize the significance of CORS, the development of authorization including bearer tokens, and the advantages of selecting optional methods in place of standard GET requests.

Why CORS Matters

In the development of APIs intended for access across various domains, CORS is essential in mitigating unauthorized access. By delineating the specific domains permitted to interact with your API, you can protect your resources from Cross-Site Request Forgery (CSRF) attacks while allowing valid cross-origin requests.

Benefits of CORS

1. Security: CORS plays a crucial role in regulating which external domains can access your resources, thereby safeguarding your API against harmful cross-origin requests.  
2. Flexibility: CORS allows you to define varying levels of access (such as methods like GET, POST, DELETE, etc.) for different origins, offering adaptability based on your specific requirements.  
3. User experience: Implementing CORS enhances user experience by allowing users to seamlessly access resources from multiple domains without encountering access-related problems.  

Before we proceed with setting up CORS, we need to understand the need to use optional methods over GET. This comparison helps in quickly comparing the aspects of using GET versus optional methods (PUT, POST, OPTIONS) in API requests.

Comparison between POST and PUT methods, the purposes and behavior:

Setting Up CORS in AWS API Gateway Using CloudFormation

Configuring CORS in AWS API Gateway can be accomplished manually via the AWS Management Console; however, automating this process with CloudFormation enhances both scalability and consistency. 

Below is a detailed step-by-step guide:

1. Define the API Gateway in CloudFormation

Start by defining the API Gateway in your CloudFormation template:

Resources:

  MyApi:

    Type: AWS::ApiGateway::RestApi

    Properties:

      Name: MyApi

2. Create Resources and Methods

Define the resources and methods for your API. For example, create a resource for /items and a GET method:

  ItemsResource:
    Type: AWS::ApiGateway::Resource
    Properties:
      ParentId: !GetAtt MyApi.RootResourceId
      PathPart: items
      RestApiId: !Ref MyApi

  GetItemsMethod:
    Type: AWS::ApiGateway::Method
    Properties:
      AuthorizationType: NONE
      HttpMethod: GET
      ResourceId: !Ref ItemsResource
      RestApiId: !Ref MyApi
      Integration:
        Type: MOCK
        IntegrationResponses:
          - StatusCode: 200
      MethodResponses:
        - StatusCode: 200

3. Configure CORS

Next, configure CORS for your API method by specifying the necessary headers:

  OptionsMethod:
    Type: AWS::ApiGateway::Method
    Properties:
      AuthorizationType: NONE
      HttpMethod: OPTIONS
      ResourceId: !Ref ItemsResource
      RestApiId: !Ref MyApi
      Integration:
        Type: MOCK
        RequestTemplates:
          application/json: '{"statusCode": 200}'
        IntegrationResponses:
          - StatusCode: 200
          	SelectionPattern: '2..'
            ResponseParameters:
              method.response.header.Access-Control-Allow-Headers: "'Content-Type,X-Amz-Date,Authorization,X-Api-Key,X-Amz-Security-Token'"
              method.response.header.Access-Control-Allow-Methods: "'*'"
              method.response.header.Access-Control-Allow-Origin: "'*'"
      MethodResponses:
        - StatusCode: 200
          ResponseModels: { "application/json": "Empty" }
          ResponseParameters:
            method.response.header.Access-Control-Allow-Headers: false
            method.response.header.Access-Control-Allow-Methods: false
            method.response.header.Access-Control-Allow-Origin: false

Incorporating Authorization

Implementing authorization within your API methods guarantees that access to specific resources is restricted to authenticated and authorized users. The AWS API Gateway offers various authorization options, including AWS Lambda authorizers, Cognito User Pools, and IAM roles.

  MyAuthorizer:
    Type: AWS::ApiGateway::Authorizer
    Properties:
      Name: MyLambdaAuthorizer
      RestApiId: !Ref MyApi
      Type: TOKEN
      AuthorizerUri: arn:aws:apigateway:<region>:lambda:path/2015-03-31/functions/<lambda_arn>/invocations

  GetItemsMethodWithAuth:
    Type: AWS::ApiGateway::Method
    Properties:
      AuthorizationType: CUSTOM
      AuthorizerId: !Ref MyAuthorizer
      HttpMethod: GET
      ResourceId: !Ref ItemsResource
      RestApiId: !Ref MyApi
      Integration:
        Type: AWS_PROXY
        IntegrationHttpMethod: POST
        Uri: !Sub arn:aws:apigateway:${AWS::Region}:lambda:path/2015-03-31/functions/${MyFunction.Arn}/invocations
      MethodResponses:
        - StatusCode: 200

After implementation, here's how the API looks in AWS:

Integration request:

API Gateway Documentation can be found here: Amazon API.

Conclusion

Establishing CORS and integrating AWS API Gateway through CloudFormation offers an efficient and reproducible method for managing API access. By meticulously setting up CORS, you guarantee that your APIs remain secure and are accessible solely to permitted origins. Incorporating authorization adds a layer of security by limiting access to only those users who are authorized. Moreover, evaluating the advantages of utilizing optional methods instead of GET requests ensures that your API maintains both security and the flexibility necessary for managing intricate operations.

The implementation of these configurations not only bolsters the security and performance of your API but also enhances the overall experience for end-users, facilitating seamless cross-origin interactions and the appropriate management of sensitive information.