Let's walk through the most likely causes and fixes for that {"message":"Forbidden"} error you're hitting—especially since your original HTTP endpoint works perfectly, so the issue is almost certainly in how API Gateway is connecting to it.

Common Culprits & Debug Steps #

1. API Gateway Doesn't Have Permission to Reach Your Backend #

• If your custom endpoint is hosted in a VPC (like an EC2 instance or Application Load Balancer), make sure you've set up a VPC Link for API Gateway. Also, check the security groups attached to your backend—they need to allow inbound traffic from the VPC Link's elastic network interfaces (ENIs).
• For public endpoints, verify your backend's firewall/WAF/security group isn't blocking API Gateway's IP ranges. You can grab the official AWS IP ranges for eu-east-1 API Gateway and add them to your allowed list.

2. Proxy Resource Configuration Glitches #

• Double-check your proxy setup:
  • Confirm the Integration Request uses the correct HTTP method (GET, POST, etc.) that matches what your client is sending. Mismatched methods often trigger forbidden errors.
  • Ensure the Endpoint URL in the integration is exactly the same as your working original endpoint—don't miss small details like http vs https, trailing slashes, or path segments. Typos here are super easy to overlook.
  • Make sure you've enabled HTTP_PROXY integration type (not the regular HTTP type). The proxy option ensures the full request path, headers, and query params get passed through to your backend correctly.

3. IAM or Authentication Misconfiguration #

• If your backend uses IAM-based authentication (like SigV4 signing), you need to configure API Gateway to sign requests with an IAM role that has permissions to access the backend. For non-AWS custom endpoints, this is less common, but if your endpoint requires API keys or auth tokens, check that API Gateway is mapping those headers correctly in the Integration Request settings.

4. Don't Skip Re-Deploying the API #

• This is a classic gotcha: after making any configuration changes, you have to re-deploy your API to the production stage you're using. Head to the API Gateway console, select your API, click "Deploy API", pick the production stage, and hit deploy. It's easy to tweak settings and forget this step!

5. Test Directly to Rule Out Client-Side Issues #

• Use curl to send a request directly to the API Gateway endpoint and see if you still get the forbidden error. This helps eliminate client-side CORS or other browser-related issues:

  curl -v https://2r3g3ttr6y.execute-api.eu-east-1.amazonaws.com/production/abc/abc.html
  

6. Backend Authorization Checks #

• If your custom endpoint has its own authorization rules, make sure API Gateway is passing any required headers or credentials. For example, if the backend expects an Authorization header, you need to map that from the client's request in the Integration Request's header mappings.

Quick Isolation Test #

Try creating a simple non-proxy GET method in API Gateway that points directly to your backend endpoint (without the proxy path). If that works, the problem is likely in the proxy resource's path mapping or integration settings. If it still returns Forbidden, the issue is definitely with permissions between API Gateway and your backend.

内容的提问来源于stack exchange，提问作者user3847894