Amazon Cognito Authorizer for Amazon API Gateway with Amazon Amplify SDK

Introduction

API security in the cloud world demands precision. Amazon Cognito authenticates users, and Amazon API Gateway protects backend services, but many implementations still misuse ID Tokens for authorization. This creates fragile, non-standard, and insecure API patterns. The correct model is straightforward: authenticate with OAuth 2.0 Authorization Code Flow, obtain Access Tokens, and let the API Gateway validate scopes and permissions. With Amplify managing token exchange and refresh behind the scenes, the entire flow becomes secure, automatic, and production aligned.

Why Access Tokens Are Mandatory for API Authorization?

ID Token → Identity

The ID Token is a profile card containing user attributes, such as email, name, phone, and sub. It is exclusively meant for client-side identity awareness, personalization, and UI logic. It must never be used to authorize APIs because it carries no permission data and was not designed for backend access control.

Access Token → Authorization

Access Tokens include scopes, client_id, group membership, the “token_use”: “access” marker, and other permission-related claims. These are what API Gateway expects. Using Access Tokens delivers:

• Clean Authorization: Bearer <token> headers
  • Strict scope-based access control
  • Proper separation between identity and authorization
  • Full alignment with OAuth2.0 standards
  • Compatibility with enterprise identity ecosystems (Google, Okta, MS Identity, Auth0)

This is the architectural rule across modern identity systems: ID Tokens tell you who, Access Tokens tell you what they can do.

Amazon Cognito OAuth2.0 Authorization Code Flow

To implement Access Token–based authorization correctly, configure your Cognito User Pool App Client with:

Authorization URL
https://your-domain-prefix.auth.region.amazoncognito.com/oauth2/authorize

Token URL
https://your-domain-prefix.auth.region.amazoncognito.com/oauth2/token

Required scopes
yourapi/read, yourapi/write

Grant Type
Authorization Code
Client Authentication
Credentials in body

After a successful authorization flow, Cognito returns:

1. Access Token
   Used by API Gateway for permission validation.
2. Refresh Token
   Enables silent session continuation.
3. ID Token
   Used for identity in the frontend.

Amazon Amplify automates redirect handling, code exchange, token storage, refresh logic, and injection of Access Tokens into API calls. No manual token juggling, no leakage issues, no brittle code.

How Amazon API Gateway Validates Access Tokens?

When a client calls an API like:

Authorization: Bearer <access_token>

The Amazon Cognito User Pool Authorizer enforces several layers of validation.

1. Signature Check

The JWT signature is validated using Amazon Cognito’s JWK set.
If the signature fails, the request is blocked instantly.

2. Token Use Enforcement

Amazon API Gateway checks the value of “token_use”.
If the client sends an ID Token, the request is rejected even if the token is valid.

3. Issuer and Expiry

Gateway verifies:

• iss matches your Amazon Cognito User Pool
  • exp is still valid
  • client_id matches the configured App Client

4. Scope Validation

Each route can require specific scopes:

/vehicles/read → fleet/read
/vehicles/update → fleet/write

If the Access Token lacks the required scope, the request is denied before your AWS Lambda function is invoked. This protects your backend from unauthorized traffic and reduces custom authorization logic in code.

Accessing User Claims Inside AWS Lambda

When a request is successfully authorized, Amazon API Gateway passes filtered claims into event.requestContext.authorizer.
AWS Lambda receives only safe, validated fields, such as:

• sub
  • scope
  • cognito:groups
  • approved custom attributes

This model prevents raw identity tokens from flowing through your backend. It avoids accidental logging of sensitive fields, reduces security exposure, and ensures consistent authorization behavior.

Best Practices for Secure Integration

• Always send Access Tokens using headers (Authorization: Bearer <token>).
  • Never log Access Tokens or ID Tokens in frontend or backend logs.
  • Keep Access Tokens short-lived and let Amplify refresh them automatically.
  • Build your authorization strategy using scopes and groups, not custom claims.
  • Enforce HTTPS everywhere.
  • Ensure server time is accurate for JWT expiry validation.

A consistent token lifecycle prevents silent failures and unexpected 401 errors in production.

Architectural Benefits of Access Token–Driven APIs

1. Strong Security Posture

Identity attributes remain in the frontend, while the backend receives only the authorization essentials. Tokens are not transmitted inside payloads or query parameters.

2. Clean API Contracts

Every route relies on standard OAuth2.0 mechanisms, not custom logic. This helps teams scale APIs without having to reinvent authorization patterns.

3. Automated Permission Boundaries

Amazon API Gateway evaluates scopes and group membership before AWS Lambda executes. This creates guardrails that reduce code complexity and security drift.

4. Enterprise Alignment

The architecture mirrors identity systems used across large organizations. It is interoperable, predictable, and easy to audit.

Practical Use Cases

Smart Home Platforms

Scopes limit access to device controls, telemetry, or sensitive operations, such as unlocking doors.

Enterprise Role-Based Systems

Admin, manager, and operator roles can be represented by Cognito groups and enforced automatically.

SaaS Multi-Tenant Systems

Access Tokens prevent cross-tenant access by embedding group or tenant-specific scopes.

IoT Control Surfaces

Operations like toggling switches or sending commands require strict, token-level permission checks.

Conclusion

Using Access Tokens for API authorization in Amazon Cognito plus Amazon API Gateway is not just a recommendation; it is the only secure, scalable, standards-oriented approach.

The result is a production-ready authentication architecture that aligns with modern OAuth practices and avoids the common pitfalls that lead to insecure APIs.

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