§01 Identity & Access

Best Practices for Designing OAuth Scopes That Do Not Over-Ask

Designing OAuth scopes is a critical aspect of any modern application that relies on third-party integrations and user permissions. Poorly designed scopes can lead to confusion, over-permissioning, and security vulnerabilities. Conversely, well-crafted scopes enhance user trust, offer clearer permissions, and minimize the risk of unintended data access. The key is aligning scopes with user-visible actions and business boundaries, rather than internal implementation details or broad catch-all permissions.

Why Scope Design Is a Product Decision

OAuth scopes establish a contract between the client and the server, articulating the types of access the client is granted. These scoped permissions inform both the consent screens that users see and the backend logic that enforces them. As such, the design of these scopes is a product decision that affects the entire user experience, not just a technical consideration.

For example, Curity recommends that scopes should include even those for endpoints that appear later in the flow. When an access token does not include the necessary scopes, APIs must return a 403 Forbidden response, and clients should enable user consent when granting scope-based access to third-party clients. Google Identity echoes this, stating that when requesting multiple scopes, apps should allow users to deny some scopes and disable corresponding features, only prompting again when appropriate and with clear reasoning.

Use Business Areas, Not Internal Structure

To create scopes that are meaningful to end users, it's essential to ground them in the business logic and user actions, not the underlying technical implementation. Scopes should describe the actual permissions being granted, rather than reflecting the internal structure of a database or API.

Connect2id recommends choosing an appropriate granularity that matches the types of access granted to a particular resource, and adding a namespace to prevent scope values from accidentally clashing. They also note that optional parameters can be included in the URI query string, and that one-time access or operations should be treated as transient consent rather than long-lived permissions.

By focusing on user-facing actions and business boundaries, scopes become more stable and less prone to "permission creep" syndrome of constantly adding and removing permissions. Curity advises considering API business areas as scopes, and only adding new scopes when a new business area is introduced.

Granularity and Namespacing

The level of granularity in scope design is crucial. Scopes should be specific enough to clearly convey the permissions being granted to the user, but not so granular that the consent screen becomes overwhelming.

Google's guidance on managing OAuth 2.0 scopes advises that scopes should reflect the actual privileges being granted, and that apps should only request the minimum necessary scopes to accomplish their goals. They also note that when requesting multiple scopes, users may deny some and disable the corresponding features, and the app should only prompt again when there is a valid reason for the additional permissions.

To avoid scope clutter and ensure clarity, it's important to namespace scopes so that they are clearly associated with specific resources or business areas. Connect2id recommends adding a namespace for each protected resource to avoid scope values accidentally matching.

Incremental Authorization and Consent Timing

OAuth scopes should also consider the timing of permission requests. Rather than asking for broad permissions upfront, it's better to grant access incrementally as it is needed.

Google's OAuth best practices emphasize this incremental approach, suggesting that apps should only request additional scopes when there is a clear need, and when the user has context for why the higher-level access is necessary. They also note that apps should avoid prompting excessively, only re-presenting scopes when the user's choice will immediately affect their ability to use the app's core features.

For transient access, Connect2id recommends treating one-time access or operations as ephemeral consent rather than long-lived permissions. This minimizes the risk of over-permissioning and ensures that users are only granting the minimum necessary access for a specific action.

Enforcement and Failure Modes

Finally, the design of OAuth scopes also impacts how permissions are enforced and the user experience when they encounter restrictions. Curity emphasizes that scopes must be enforced at every API endpoint, and that APIs should return a 403 Forbidden response when an access token lacks the necessary permissions.

When a user denies a required scope, Google Identity states that the app should gracefully handle this by disabling the corresponding features or gracefully deprecating the feature. This ensures a smoother user experience, while still clearly conveying the impact of their permission choices.

Conclusion

In conclusion, designing user-facing OAuth scopes is a critical consideration for any modern application that relies on third-party integrations and user permissions. By thinking user-centrically, wrapping permissions in business logic, and implementing granular and context-appropriate scopes, you provide an app experience of transparency and trust to users and operational clarity and safety to your API installation. The best practices shared here—aligning scopes with user actions, using business boundaries, assuming incremental permissioning, and enforcing them strictly—offer a framework for building a more secure, user-centric approach, setting up a better future for OAuth scope design.