As applications and user bases grow more complex, so must the access policies governing them. One powerful mechanism for implementing fine-grained access control is the use of claims. In this article, we take a look at what claims are and how to implement them to secure your APIs effectively.
What are Claims?
Claims are part of tokens that provide information about an entity or granted access. In particular, in OpenID Connect, an ID token can contain claims with information about the user, such as their first and last names, their role, or the region they are based in. Claims are asserted by the asserting party, which states that the subject has some attribute. The asserting party is usually an identity provider (IdP). Essentially, claims are assertions that allow applications and APIs to trust attributes about a user.
Why Use Claims for Fine-Grained Access Control?
A common approach for providing access control is to base it on a user’s role. While roles offer a simple way to group users and assign permissions, they are often too static and coarse-grained for today’s complex applications. Claims, on the other hand, are dynamic key-value pairs issued by an identity provider that describe various attributes of a user, such as department, region, clearance level, or authentication context. Unlike roles, claims provide rich, contextual information that allows access decisions to be based not just on who the user is, but on how, where, and under what conditions they are accessing a resource.
Elevating access control from basic role checks to claims-based authorization is essential for implementing fine-grained, context-aware policies that align with a zero-trust architecture. Some scenarios where using claims is essential include:
Multi-tenant architectures, where each tenant needs isolated access to its own resources.
Distributed teams, where access must vary across departments or regions.
Attribute-based access control (ABAC) needs, where access is determined by multiple attributes beyond just roles.
Claims allow services to evaluate access dynamically based on the context provided at runtime. For example, an API can use the department claim to ensure only users from "marketing" can access specific endpoints or the tenant_id claim to verify ownership of a resource. This makes authorization more flexible, scalable, and secure, especially in zero-trust environments where contextual awareness is crucial.
How to Implement Claims
Before implementing claims, determine what attributes are relevant for access decisions. An attribute is minimally defined as a name/value pair, specifying some attribute and the value for that attribute. These attributes contain the information needed to form a claim. Consider what resources need to be protected, who can access what, and under what conditions. In addition, you should think about the contextual factors that influence access, such as the user’s role, department, or geographic location. From this, derive a schema for the claims you need, such as tenant_id, access_level, resource_owner, etc.
Next, you should use your identity provider to issue these claims. This includes collecting user attributes during authentication and mapping them to claims. For example, in the Curity Identity Server, this may involve defining attribute sources (authentication, account, external APIs), creating claims rules, and associating claims with scopes in the Claims Mapper.
Once the client requests authentication with specific scopes, the IdP issues a token containing claims that correspond to these scopes. Apart from any user-related attributes, this can also mean the contextual data that APIs will later rely on.
On the API side, tokens are validated by verifying their signature and checking standard claims like exp (expiration time), iss (issuer), and aud (audience). Then, custom claims are used in business-level access control logic. For example, you might check that a department claim matches the expected value before allowing access.
Claims-based policies can be enforced inline in code or externally using policy engines like OPA (Open Policy Agent), depending on the complexity and scalability needs of your system.
Finally, claims usage should be monitored and audited. Log access decisions and token contents for visibility, and continuously evaluate whether your claims model still fits your application’s security posture. Claims-based access control is an evolving process, not a one-time configuration.
Ensure that access decisions are logged with claim context for auditability. Monitor tokens and access patterns to:
- Detect abuse or misconfiguration
- Tune claim granularity and scope boundaries
- Improve policy accuracy
Claims implementation is not a one-off task. It’s an iterative design that evolves with application complexity and security posture.
Conclusion
Claims provide a flexible, context-rich foundation for fine-grained access control in APIs. When implemented thoughtfully, they enable attribute-based policies that scale across teams, tenants, and geographies. By shifting authorization logic to leverage claims, API developers can secure services with higher precision, without locking into rigid roles or complex ACLs.
Ready to upgrade your API authorization? Start by identifying the claims that matter most in your domain, and enforce them smartly where it counts.
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