How can you customize the claims returned in an ID Token ?

Customizing claims in an OpenID Connect (OIDC) ID Token is crucial for tailoring user information to your application’s specific needs. The primary methods involve configuring claims directly within your Identity Provider (IdP), fetching supplementary data from the UserInfo endpoint, or mapping incoming claims client-side within your application. For more advanced scenarios, custom scopes can provide granular control over which claims are released.

The ID Token, a core component of OpenID Connect, is a JSON Web Token (JWT) that carries verified identity information about an end-user. While it includes standard claims like sub, name, and email, many applications require additional, custom data—such as roles, permissions, or tenant IDs—to manage user access and personalize experiences. Understanding how to effectively customize these claims is fundamental for building robust and efficient authentication and authorization systems.

Core Methods for Customizing ID Token Claims

1. Identity Provider (IdP) Configuration (Standard & Custom Claims)

Configuring claims directly at the Identity Provider (IdP) is often the most straightforward and authoritative method. This is where claims are generated and added to the ID Token before it’s issued to the client application.

• Standard Claims: These are the foundational claims (e.g., sub, name, email) that provide basic user identification. The sub (subject) claim is particularly important as it uniquely identifies the end-user. These are typically included by default based on requested scopes (e.g., openid, profile, email). Standard claims form the bedrock of user identification. The sub claim is crucial, acting as the unique identifier for the user. name and email provide human-readable identification and communication details, essential for basic user display and interaction within the application.
• Custom Claims: To enrich the user context with application-specific data, you can inject custom claims directly at the IdP level. This might include roles, permissions, tenant_id, or department. These empower you to add rich contextual information to the ID Token. Many IdPs provide mechanisms for this:
  • Auth0: Uses “Rules” (JavaScript functions executed during authentication) to add, modify, or remove claims.
  • Azure AD: Supports “Application Roles” which can be assigned to users and automatically included as claims in the ID Token. Custom claims can also be configured via “Token Configuration.”
  • Keycloak: Allows for “Mappers” to define how user attributes or role memberships are transformed into claims.

  Example: In our SaaS platform, we needed to include the user’s “organization ID” and their “role” (admin, editor, viewer) within the application. We achieved this by using Auth0 Rules to inject these claims directly into the ID Token based on the user’s database profile. This eliminated the need for separate database lookups after login, improving performance.

2. UserInfo Endpoint for Additional Claims

The OIDC UserInfo endpoint provides a standardized way to retrieve additional claims about the authenticated end-user. Unlike claims embedded directly in the ID Token, these are fetched via a separate API call using the access token.

• Benefits:
  • Reduces ID Token Size: Keeps the ID Token compact, optimizing initial login speed and reducing network overhead, especially for clients with limited bandwidth (e.g., mobile devices).
  • Flexible Data Retrieval: Allows for less frequently accessed or larger claims (e.g., profile_picture_url, last_login_timestamp) to be retrieved on demand.
• Considerations:
  • Performance Trade-off: Introducing an extra API call can slightly increase overall authentication latency. This trade-off needs to be carefully balanced.
  • Caching: Client applications can cache UserInfo responses to mitigate the performance impact of repeated calls.

  Example: The UserInfo endpoint serves as a secondary source for claims, especially for larger or less frequently used claims. In a previous project, we offloaded user attributes like “profile picture URL” and “last login timestamp” to the UserInfo endpoint. This kept the ID Token compact, optimizing login speed, but it did introduce an extra API call. We had to carefully balance this trade-off.

3. Client-Side Claim Mapping

After receiving the ID Token, your application can internally map or transform the incoming claims to better suit its internal data structures, naming conventions, or authorization logic. This doesn’t modify the token itself but adapts its contents for application use.

• Use Cases:
  • Renaming Claims: Mapping an incoming department claim to an internal team property.
  • Normalizing Values: Converting claim values (e.g., is_admin: 1 to role: 'admin').
  • Deriving Properties: Creating new application-specific properties based on a combination of claims.

  Example: Client-side mapping is useful for adapting claims to your application’s specific needs. For instance, our ID Token contained a “department” claim, but our application used a different internal naming convention, “team.” We simply mapped the “department” claim to the “team” property during token handling. This allowed us to use consistent terminology throughout our codebase.

4. Custom Scopes (Advanced Control)

While more advanced, defining custom scopes can provide fine-grained control over which specific claims are included in the ID Token or made available via the UserInfo endpoint. When a client requests a custom scope, the IdP can be configured to trigger specific logic to release a predefined set of claims associated with that scope.

• Benefits:
  • Granular Authorization: Clients only receive claims relevant to the specific access they’re requesting.
  • Conditional Claim Release: Allows for claims to be included only when a particular access pattern is required.

  Example: Custom scopes offer fine-grained control over claim release. While more complex, we explored custom scopes when integrating with a legacy system. We defined a scope called “legacy_access” which, when requested during authentication, triggered a specific rule in our IdP to include a legacy user ID required for integration. This allowed us to manage claim distribution based on specific access requirements.

Strategic Considerations for Customizing ID Token Claims

Understand Standard vs. Custom Claims and Use Cases

Demonstrate a clear understanding of standard claims versus custom claims and their use cases. Explain when you would add custom claims for role-based access control or multi-tenancy scenarios.

Interview Insight: “In a recent project building a multi-tenant application, we utilized custom claims for role-based access control and tenant isolation. Standard claims like sub identified the user, but we added custom claims like tenant_id and roles (e.g., ‘admin’, ‘editor’) directly within the ID Token using Keycloak’s role mapping feature. This streamlined authorization checks within the application – every request had the necessary context, preventing additional database calls to determine user roles or tenant association. This significantly improved performance and simplified our authorization logic.”

Explain Trade-offs: ID Token Size vs. UserInfo Endpoint

Explain the trade-offs of including claims directly in the ID Token versus fetching them from the UserInfo endpoint. Discuss balancing ID Token size against additional round trips. Show you understand performance implications.

Interview Insight: “We faced this exact dilemma when designing our user authentication flow. Initially, we included all user details, including profile information and settings, directly in the ID Token. However, this resulted in a large token size, impacting page load times, especially on mobile devices. We then moved less frequently accessed data, like profile_picture and last_login, to the UserInfo endpoint. This optimized the ID Token size, improving initial login speed. We acknowledged the extra round trip to the UserInfo endpoint, but we deemed it acceptable given the performance gains from a smaller ID Token. We used caching strategies to mitigate the performance impact of fetching user info.”

Mention Experience with Specific IdPs and Mechanisms

If you have experience with a specific IdP (like Auth0, Azure AD, Keycloak), mention its specific mechanisms for adding custom claims (e.g., Auth0 Rules, Azure AD Application Roles).

Interview Insight: “We use Azure AD extensively. When we needed to add a ‘department’ claim, we leveraged Azure AD Application Roles. We defined the different departments as application roles and assigned users to these roles. This automatically included the assigned roles as a claim in the ID Token during authentication, simplifying user management and authorization within our application.”

Discuss Security Implications of Custom Claims

Be prepared to discuss the security implications of adding custom claims and how to avoid exposing sensitive data unnecessarily.

Interview Insight: “Absolutely. We are very mindful of security. We follow the principle of least privilege – only include the minimum necessary claims in the ID Token. We avoid adding sensitive information like social security numbers or credit card details. When adding custom claims, we validate the data source and sanitize the data to prevent injection attacks. We also use short, descriptive claim names to minimize token size and improve readability. For highly sensitive data, we explore alternative approaches like secure data stores accessed via separate APIs after authentication, rather than embedding them in the ID Token.”

Conclusion

Customizing claims in an OIDC ID Token is a powerful capability that allows applications to receive precisely the user information they need for authentication, authorization, and personalization. By strategically utilizing Identity Provider configurations, the UserInfo endpoint, client-side mapping, and advanced custom scopes, developers can design secure, efficient, and tailored identity solutions that meet diverse application requirements.