Troubleshooting Keycloak Refresh Token azp Mismatch: A Backend-Centric Solution

Introduction:

In the realm of user authentication and authorization, Keycloak has made a name for itself as an open-source Identity and Access Management solution. It boasts extensive capabilities, such as OAuth 2.0, OpenID Connect, and SAML. However, during the process of integration with applications, certain challenges might occur. This article seeks to explain a specific issue related to azp mismatch during refresh token exchange, along with an effective solution.

Understanding the azp claim:

The azp claim in OAuth 2.0 tokens refers to the authorized party, i.e., the client ID of the application for which the token is issued. When exchanging a refresh token for an access token, the client associated with the refresh token must be identical to the client making the request.

Scenario:

Let’s set a context. Suppose a spring-boot-admin-client performs naked impersonation to issue tokens due to a already exists social login context , and a separate client, such as a mobile-app-client, attempts to refresh these tokens. You might encounter an error message:

{ 
  "error":"invalid_grant", 
  "error_description":"Invalid refresh token. Token client and authorized client don't match" 
}

This error is indicative of an azp mismatch. In simpler terms, the client associated with the refresh token does not match the client requesting the refresh.

Solution:

The solution requires some backend manipulation. Instead of exposing the spring-boot-admin-client secret to the frontend, the backend service performs an inspection of the azp field in the incoming refresh token and adjusts the client accordingly before forwarding the request to Keycloak.

Here’s an outline of the process:

1. The frontend application sends a request for token refresh to the backend service, using the mobile-app-client credentials.

2. Upon receiving the request, the backend service decodes the incoming refresh token to inspect the azp field.

• If the azp matches the mobile-app-client, the backend service leaves the request as is.
• If the azp matches the spring-boot-admin-client, the backend service replaces the mobile-app-client credentials in the request with the spring-boot-admin-client credentials.

3. The backend service forwards the updated request to Keycloak.

4. Keycloak returns the new tokens to the backend service.

5. Finally, the backend service securely forwards the new tokens back to the frontend application.

With this approach, you ensure the client secret is kept secure, not exposed to the frontend, and is still usable to refresh tokens.

Conclusion:

Keycloak, as robust as it is for handling authentication, does have complexities that require detailed understanding and correct handling. The azp mismatch issue is a perfect example. By implementing this backend-centric solution, this issue can be effectively addressed.