Introduction

AWS API Gateway returns 403 Missing Authentication Token when the request does not match a deployed resource and method combination at the URL you are calling. Despite the wording, this error often has nothing to do with IAM or authorizers. It usually means the path, stage, custom domain base path mapping, or HTTP method does not line up with what API Gateway has actually deployed.

Symptoms

  • Requests to an API Gateway endpoint return 403 Missing Authentication Token immediately.
  • The API works in the console test tool but fails from curl, frontend code, or external clients.
  • A custom domain endpoint fails while the default execute-api URL works.
  • One method such as POST fails while another path or method continues to succeed.
  • The error started after deploying a new stage, changing routes, or updating domain mappings.

Common Causes

  • The client is calling the wrong URL path or omitting the stage segment.
  • The API change was not deployed to the stage you are testing.
  • A custom domain base path mapping points to the wrong API or wrong stage.
  • The HTTP method used by the client is not defined for that resource.
  • A trailing slash, rewritten path, or proxy integration mapping changes the final route unexpectedly.
  • The request is hitting a deleted or renamed resource that still exists in client code, documentation, or cached configuration.

Step-by-Step Fix

  1. Copy the exact failing request URL and method from the client. Compare it with the deployed API Gateway route configuration instead of relying on assumptions from memory or documentation.
  2. Verify whether you are using the default execute-api hostname or a custom domain. If you use a custom domain, confirm the base path mapping points to the expected API and stage.
  3. Check the resource and method definitions in API Gateway. Make sure the path exists exactly as called and that the HTTP method in the client matches the deployed method.
  4. Confirm the latest API changes were deployed to the stage you are testing. In many cases the route exists in the API definition but has not been deployed yet.
  5. Test the same route against the execute-api URL and then against the custom domain. If one works and the other fails, focus on domain mapping, path rewriting, or CDN and reverse proxy behavior in front of API Gateway.
  6. Review any frontend router, reverse proxy, or load balancer rewriting rules. A missing stage prefix or an extra path segment is a common reason the request misses the intended route.
  7. If you use greedy proxy resources such as /{proxy+}, confirm the integration and stage deployment still support the path you are sending.
  8. Re-test with curl using the expected method, headers, and full path. This isolates route mismatches from browser behavior or application-side URL generation bugs.
  9. After correcting the route, stage deployment, or domain mapping, call the endpoint again and confirm the request now reaches the integration instead of failing at API Gateway routing.
  10. Finish by checking access logs and application logs so you know the request is flowing through the intended method and backend.