Amazon Cognito is a service provided by AWS that enables developers to manage user authentication, authorization, and user data for their applications. As with any authentication service, understanding the structure and intent behind tokens, especially access tokens, is essential for securely implementing these systems. A specific point of consideration for developers working with Amazon Cognito is the absence of the aud (audience) claim in its access tokens. This article delves into the reasons for this absence and provides insights into its implications.

Understanding Access Tokens and the Audience Field

JWT Token Structure

JSON Web Tokens (JWT) are commonly used for authentication and data exchange. A standard JWT consists of three parts: header, payload, and signature.

• Header: Typically consists of two parts: the type of token (JWT) and the signing algorithm (e.g., HMAC SHA256).
• Payload: Contains the claims. Claims are statements about an entity (typically, the user) and additional data.
• Signature: Used to verify the token's integrity.

Claims in JWTs

Claims are the core part of a JWT, providing information about the token subject. They fall into three categories:

1. Registered Claims: Predefined and recommended claims, including iss (issuer), sub (subject), aud (audience), exp (expiration time), and others.
2. Public Claims: Custom claims agreed upon between parties for information exchange.
3. Private Claims: Custom claims defined at will, used uniquely between systems where the JWT is exchanged.

The aud

Claim

The aud (audience) claim specifies the intended recipient of the token. Its purpose is to ensure that the token is used by a specific audience and not by others. Typically, clients validate the audience by checking the aud claim against their own identifiers.

Amazon Cognito's Decision to Omit aud

in Access Tokens

Differentiation Between Access and ID Tokens

One of the essential aspects of Amazon Cognito is the differentiation between access tokens and ID tokens:

• Access Tokens: Used to authorize API requests which gain access to user data or resources.
• ID Tokens: Used to verify user identity and acquire basic user profile information.

Amazon Cognito's access token serves a different purpose than the ID token. For access tokens, Cognito's primary use is to authorize resources in AWS and scope access levels, rather than to validate identities.

Security Model Considerations

By design, Amazon Cognito aligns access tokens with resource servers rather than client applications. Unlike ID tokens, where the client needs to validate the user identity and target specific applications, the primary concern for access tokens is ensuring the appropriate level of access to resources.

Including an aud claim might suggest that clients are responsible for validating that a given request intending to use the token is performed by the right application. In reality, the trust and validation flow involving access tokens have a broader context, typically relying on scopes.

Simplifying Authorization

With the aud claim omitted, Amazon Cognito simplifies the model used for authorizing access to resources. Access is controlled by scopes included in the token and the resource server, rather than through additional audience validation at the client or application level.

Practical Implications

Handling Access Tokens in Resource Servers

Resource servers are responsible for validating access tokens and ensuring that they match the permission scopes required to access specific resources. By using scopes, developers ensure that permissions are properly managed without needing to validate the audience.

Example Code

Here’s a simple example of how a resource server might validate an access token using scopes with the AWS SDK: