Introduction

Amazon Web Services (AWS) API Gateway is a powerful service that allows developers to create, maintain, and secure APIs at any scale. However, users can occasionally encounter permission-related issues, such as the error message: "User anonymous is not authorized to execute API." This article explores the components, configurations, and troubleshooting steps related to this authorization error.

Understanding the Components

API Gateway

AWS API Gateway acts as a front door for applications to access data, business logic, or functionality from backend services like AWS Lambda functions, EC2 instances, or HTTP APIs.

IAM Roles and Policies

Permissions in AWS are managed using Identity and Access Management (IAM). When you configure permissions, you must create policies that explicitly allow users to call your API methods.

Authorization Mechanisms

1. IAM Permissions: Used to manage access to AWS resources.
2. Lambda Authorizers: Custom AWS Lambda functions to control access to APIs.
3. Cognito User Pools: Provides federated identities by leveraging social logins or SAML.
4. API Keys: A straightforward way to control access with little overhead.

Common Causes of Authorization Errors

Missing IAM Policy

The most common cause of "User anonymous is not authorized to execute API" is a missing or incorrectly configured IAM policy. Ensure the policy attached to the user or role includes permissions for `execute-api:Invoke`.

Incorrect Resource Path

Check if the `Resource` field in your IAM policy is explicitly allowing the method and resource path.

CORS Issues

Cross-Origin Resource Sharing (CORS) can lead to invocation issues if not correctly configured, especially for APIs accessed from web browsers.

Troubleshooting Steps

To resolve the "User anonymous is not authorized to execute API" error, follow these steps:

1. Verify API Method Settings:
   • Check if the method has "AWS_IAM" as its authorization type if you are using IAM authorizations.
2. Check Associated IAM Policies:
   • Ensure the IAM user or role making the request has correct permissions.
   • Policy example:
   • Use the IAM Policy Simulator to validate the permissions granted to your IAM user or role.
   • Enable logging for better visibility. Use CloudWatch Logs to see detailed request information.
   • Ensure the API Gateway method is configured correctly for CORS if accessed from a browser.
   • Use tools like Postman or `cURL` to test API calls with credentials to isolate issues.
   • If using a Lambda Authorizer, make sure it returns a correct IAM policy denying or allowing access.