How to Use OAuth Scopes for Authorization

Why OAuth is Important

OAuth2 is a vital protocol for the authorization and authentication of web applications, allowing users to grant controlled access to their resources without exposing their personal credentials​​. Despite its widespread adoption, there's a common misconception among developers regarding the use of OAuth scopes for authorization.

Our goal in this blog is to explain the intended use of OAuth scopes, highlight the problems with their misuse for authorization, and suggest better alternatives.

What are OAuth Scopes for?

OAuth scopes serve as permissions in the OAuth 2.0 framework, defining the level of access an application has to a user's account. Each scope represents a specific action, like reading emails or managing files. This selective access control is integral to user privacy and security​​. 

Examples of OAuth scopes in popular services like Slack, GitHub, and Google show their varied applications. For instance, Google uses distinct scopes for different services like Gmail or Google Drive, ensuring applications access only the necessary parts of a user's account​​​​.

However, OAuth scopes were never designed to be a comprehensive authorization mechanism. They provide a way to limit access but not to specify or enforce detailed user permissions within an application. This distinction is often overlooked, leading to misuse and potential security risks​​.

Authorization with OAuth Scopes

OAuth scopes are initially attractive for authorization because they seem to offer a straightforward way to differentiate user roles, like 'Read' and 'Write'. For instance, a basic implementation might involve creating a scope like ‘Read’, which is checked in every request to ensure proper access​​. However, this approach is oversimplified and does not scale well with the growing complexity of applications.

As an application evolves, the binary distinction of 'Read' and 'Write' proves insufficient. The limitations of OAuth scopes become apparent when more nuanced access controls are needed, such as differentiating between 'Read' and 'Write' permissions or accessing specific parts of a user's data​​​​.

Why not use Scopes to Make Authorization Decisions

The misuse of OAuth scopes for detailed authorization decisions can lead to several problems:

Over-Permissioning

Over-permissioning occurs when applications are granted broad scopes that allow excessive access to user data. For example, an application with an 'Admin' scope might gain access to sensitive areas of a user's account that it doesn’t need for its functionality, such as private messages or personal information. This excess permission increases the risk of data breaches and compromises user privacy, as the application can access more information than necessary for its intended use.

Scalability Issues

As applications evolve, the management of numerous OAuth scopes becomes cumbersome. Imagine a third-party healthcare app that needs access to a medical platform. As the platform grows, it may need to offer finer distinctions between different types of data, like patient records, appointment schedules, and billing information. The app would require a multitude of specific scopes to access these different data types, making it difficult to manage and maintain clear access control and potentially leading to unnecessary data exposure.

Lack of Adaptability

OAuth scopes are rigid and do not adapt well over time. For example, if a social media platform adds a new privacy feature, the existing 'user-profile' scope might not adequately restrict or allow access to this new feature. This inflexibility means that once a scope is granted, it remains in effect until explicitly revoked, which can be problematic as applications evolve and their features and access requirements change.

Context-Specific Access Needs

Scopes often fail to accommodate varying access levels required in different contexts. In a file-sharing application, for instance, a user granted 'read-write' access might incorrectly have the same level of access to all files, regardless of their sensitivity or ownership. This issue can lead to users being over-privileged or under-privileged, depending on the situation, and poses significant risks to data security and integrity.

Complex Policy Support

Utilizing OAuth scopes for complex policy support can create security vulnerabilities. A banking application that uses simple scopes without considering nuances such as transaction limits or specific user roles may enable unauthorized high-value transactions. OAuth scopes are not designed to handle such complex, dynamic access control policies, which often require considerations beyond the scope of what OAuth was intended to handle.

The Real Usage of Scopes for Authorization - Role Assignment

Given the challenges outlined above, it's evident that OAuth scopes need to be confined to a more limited role within the broader authorization system. The problems we've discussed highlight the necessity to set boundaries on what scopes can and cannot do. This constraint is crucial to ensure that they are not misapplied for purposes they are not designed to handle, such as detailed, context-specific authorization decisions.

Comparing this to role assignment, OAuth scopes should be analogous to setting up basic user roles during the configuration phase of an application. Just as roles like 'editor', 'viewer', or 'admin' are established to define broad access levels, scopes should similarly define the basic capabilities of an application or user. This approach aligns with the original intent of scopes in OAuth - to grant external applications a defined level of access, rather than serving as a tool for making complex, real-time authorization decisions.

Use Policy-Based Configuration for Authorization, Incorporate OAuth Scopes

The development of a dynamic authorization system requires a more detailed approach that goes beyond static role assignments. A modern, robust system should be built on policies as code and a flexible permissions model such as RBAC, ABAC, or ReBAC. This approach allows for the creation of granular, context-aware rules that can adapt to the evolving needs and complexities of an application. Policy-based systems enable real-time decisions based on a variety of factors, such as user attributes, resource types, and environmental conditions, ensuring a secure and efficient access control mechanism.

Example of non-scalable imperative scope authorization 👇🏻

if (scope != 'read') {
    return;
}

Example of dynamic policy-based scope check 👇🏻

permit.check({
  // the user key
  key: "john@permit.io",
  // just-in-time attributes on the user
  attributes: {
    scope: scope,
  },
}, 'access', 'sheet:non_accesible_by_readers');

Incorporating OAuth scopes into this framework complements the dynamic authorization system. Scopes serve as an initial layer of access control, defining the broad boundaries within which more nuanced decisions are made. They ensure that external applications or services are restricted to appropriate levels of user data access. Within these boundaries, the policy-based system takes over, utilizing detailed rules and permissions to make fine-grained authorization decisions. This layered approach effectively leverages the strengths of both OAuth scopes and policy-based authorization, providing a comprehensive and secure solution for managing access control.

Conclusion

The approach developers take to authorization needs to be reevaluated. Instead of relying solely on OAuth scopes, it's advisable to use comprehensive solutions that offer a permission management system that simplifies the implementation of complex authorization mechanisms, ensuring applications remain secure and adaptable while keeping OAuth scopes in their intended role as access-level definers​.

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