Authorization Demystified: Best Practice #3 for Securing REST APIs with API Gateway
Securing REST APIs is foundational in building modern, scalable, and secure applications. When paired with strong authorization practices, Amazon API Gateway becomes a robust tool for protecting your backend services from unauthorized access. In this guide, we’ll demystify Best Practice #3: Implementing Authorization and explore how to effectively apply it using API Gateway to secure your REST APIs.
Understanding Authorization vs. Authentication
Before diving in, it's essential to differentiate between authentication and authorization:
Authentication confirms who you are.
Authorization defines what you're allowed to do once authenticated.
While authentication verifies identity (e.g., via Cognito, OAuth), authorization ensures that authenticated users can only access permitted resources or actions.
Best Practice #3: Use Fine-Grained Authorization Mechanisms
Here’s how to implement secure, scalable, and maintainable authorization in API Gateway:
1. Leverage AWS IAM for Internal Services
When your APIs are accessed by internal AWS services (like Lambda, EC2, or another API), use AWS IAM roles and policies for authorization. This avoids exposing credentials and enforces least-privilege access.
2. Enable Amazon Cognito User Pools for User Authorization
For applications that require user-based access control:
Use Cognito User Pools to manage user sign-up/sign-in and token issuance.
Attach resource policies to the API Gateway that validate JWT tokens.
Use Cognito groups or custom claims for role-based access control (RBAC).
Example:
{
"Effect": "Allow",
"Principal": "*",
"Action": "execute-api:Invoke",
"Resource": "arn:aws:execute-api:*:*:*/*/GET/user-profile",
"Condition": {
"StringEquals": {
"cognito-identity.amazonaws.com:sub": "${userId}"
}
}
}
3. Use Lambda Authorizers for Custom Logic
When Cognito or IAM isn’t flexible enough, implement a Lambda Authorizer (formerly known as Custom Authorizer) to handle:
Token validation
Role and permission evaluation
Multi-tenant validation logic
Your Lambda authorizer returns a policy document that defines allowed or denied actions.
4. Apply Resource Policies for Static Controls
Use resource policies to restrict API access based on:
Source IP ranges
VPC endpoints
AWS accounts or IAM principals
These are ideal for network-level authorization on sensitive APIs.
Combine Layers for Maximum Security
Use a defense-in-depth approach:
Authenticate with Cognito or OAuth.
Authorize with IAM, Lambda authorizers, or group-based access control.
Restrict access with resource policies.
Testing and Monitoring
Use CloudWatch Logs to trace authorization failures.
Implement AWS WAF for rate limiting and additional rules.
Conduct regular pen tests and least-privilege reviews.
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