AWS API Gateway: Resolving 403 Errors on OPTIONS Requests During SAM Local Invocation

Understanding 403 Errors on Local AWS API Gateway

After working with AWS API Gateway and testing locally through AWS SAM (Serverless Application Model), it's typical to discover bugs that don't arise after the API is deployed. One issue is obtaining a 403 Forbidden error when executing the OPTIONS method, despite properly configuring the API for CORS and setting the AuthorizationType to "NONE". This problem can be especially aggravating when the setup runs smoothly in the deployed environment.

When testing OPTIONS requests locally with curl, the API Gateway may return a "Missing Authentication Token" error. This is perplexing because the OPTIONS method should not require authentication, especially when expressly set to deliver a 200 result. Identifying the source of this disparity is critical for successful local development.

Understanding why SAM local behaves differently than the deployed API Gateway can help you troubleshoot this issue. It is critical to delve into the configuration details and ensure that the local and deployed environments match as nearly as feasible. Misconfigurations frequently result in such errors.

In this article, we will look at the probable causes of the 403 error during local development and how to address it. We’ll review common pitfalls in SAM templates, CORS handling, and API Gateway setups, so you can avoid these hurdles and continue building effectively.

Understanding and Optimizing AWS API Gateway Local Invocation

In this article, we will look at the probable causes of the 403 error during local development and how to address it. We’ll review common pitfalls in SAM templates, CORS handling, and API Gateway setups, so you can avoid these hurdles and continue building effectively.

In the Express server, we use res.setHeader() to set CORS headers like "Access-Control-Allow-Origin" and "Access-Control-Allow-Methods". This ensures that the appropriate headers are returned to the client, allowing for cross-origin requests. Additionally, the script's POST method connects to an AWS DynamoDB table via the AWS SDK. The scan operation is a read-only action that returns all records from the chosen table, allowing us to test database interactions locally. Proper error handling is used to manage database connection issues, ensuring that the server responds appropriately to failures.

The second example, built in Python with Flask, provides the same functionality as the Node.js script but is intended for developers who are more experienced with Python. Flask's app.route() method routes both the OPTIONS and POST methods to specified routines, ensuring that CORS preflight requests are handled easily. Custom responses are defined using the app.response_class() method, which includes the relevant CORS headers. The POST method, like the Node.js example, uses the AWS SDK for Python (boto3) to scan a DynamoDB table. This modularity allows developers to simply alter the backend based on whether they prefer JavaScript or Python.

Finally, the SAM template setup ensures that the AWS API Gateway is appropriately set up to receive POST and OPTIONS queries. The PassthroughBehavior attribute is set to "WHEN_NO_MATCH", which allows the API Gateway to handle requests that do not match a predetermined template. This is useful when working with mock integrations since it allows the system to deliver a 200 status code without really running a backend Lambda. The IntegrationResponses and MethodResponses sections specify the headers and response parameters that ensure the OPTIONS method sends a correct CORS configuration to the client. This method is crucial for avoiding the "403 Forbidden" issue during local SAM tests.

// Import required modules
const express = require('express');
const AWS = require('aws-sdk');
const cors = require('cors');
const app = express();
app.use(cors());
// Middleware for JSON request parsing
app.use(express.json());
// CORS preflight response handling
app.options('/scanRecords', (req, res) => {
  res.setHeader('Access-Control-Allow-Origin', '*');
  res.setHeader('Access-Control-Allow-Methods', 'POST, OPTIONS');
  res.setHeader('Access-Control-Allow-Headers', 'Content-Type, Authorization');
  res.status(200).send();
});
// Main POST method for scanRecords
app.post('/scanRecords', async (req, res) => {
  const dynamoDB = new AWS.DynamoDB.DocumentClient();
  try {
    const params = { TableName: 'RecordsTable' };
    const data = await dynamoDB.scan(params).promise();
    res.status(200).json(data);
  } catch (err) {
    res.status(500).send('Error fetching records');
  }
});
// Start the Express server on PORT 3000
app.listen(3000, () => {
  console.log('Server running on http://localhost:3000');
});

from flask import Flask, jsonify, request
import boto3
app = Flask(__name__)
# CORS headers for OPTIONS requests
@app.route('/scanRecords', methods=['OPTIONS'])
def options_method():
    response = app.response_class(status=200)
    response.headers['Access-Control-Allow-Origin'] = '*'
    response.headers['Access-Control-Allow-Methods'] = 'POST, OPTIONS'
    response.headers['Access-Control-Allow-Headers'] = 'Content-Type, Authorization'
    return response
# POST method to scan records from DynamoDB
@app.route('/scanRecords', methods=['POST'])
def scan_records():
    dynamodb = boto3.resource('dynamodb')
    table = dynamodb.Table('RecordsTable')
    try:
        response = table.scan()
        return jsonify(response['Items']), 200
    except Exception as e:
        return str(e), 500
# Run the Flask app on port 3000
if __name__ == '__main__':
    app.run(debug=True, host='0.0.0.0', port=3000)

Resources:
  scanRecords:
    Type: AWS::Serverless::Function
    Properties:
      Handler: dist/dynamo/CRUD.scanRecords
      CodeUri: ./backend
      Policies:
        - AmazonDynamoDBFullAccess
        - CloudWatchLogsFullAccess
      Events:
        ApiEvent:
          Type: Api
          Properties:
            Path: /scanRecords
            Method: post
  scanRecordsOptionsMethod:
    Type: AWS::ApiGateway::Method
    Properties:
      AuthorizationType: NONE
      HttpMethod: OPTIONS
      ResourceId: !Ref scanRecordsResource
      RestApiId: !Ref apiGatewayRestApi
      Integration:
        Type: MOCK
        IntegrationResponses:
          - StatusCode: 200
            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: "'POST,OPTIONS'"
              method.response.header.Access-Control-Allow-Origin: "'*'"

Troubleshooting AWS API Gateway Local 403 Errors

Understanding how CORS (Cross-Origin Resource Sharing) policies are enforced in the API Gateway is crucial when seeing a 403 error during a SAM local invoke. While your deployment may handle CORS appropriately on the cloud, local invocation using AWS SAM can sometimes result in incompatibilities between how the OPTIONS method is handled. This is because local environments may not always accurately duplicate all setups, and the OPTIONS mechanism must be properly integrated to avoid authentication difficulties.

Another key feature is that the 403 error is frequently associated with missing or incorrectly configured API Gateway permissions. During local development, it's critical to ensure that your SAM template appropriately defines AuthorizationType as "NONE" for OPTIONS requests, and that the corresponding permissions in the Lambda function are properly set up. Otherwise, the request will return a "Missing Authentication Token" message, indicating that the system expects an authentication mechanism that was not specified.

Finally, handling mock integrations is an efficient technique to avoid the requirement to call the Lambda function for the OPTIONS method. Create a MOCK integration with response parameters in your API Gateway to guarantee the OPTIONS method delivers a default 200 response with the required CORS headers. This simplifies the development process and helps to avoid 403 errors, which are frequently caused by unmanaged preflight queries in both local and deployed settings.