To determine if your JSON Web Token (JWT) flow is correctly configured from the Client to the Backend for Frontend (BFF) to the Resource Server, you should ensure that the following steps are properly implemented:
JWT Issuance: The flow begins with issuance of the JWT. This usually involves the client authenticating with an authentication server (which could be a part of the BFF or external to it), after which the server issues a JWT. Ensure that the token is signed with a robust algorithm such as RS256 or HS256 depending on your security requirements.
Client-side Token Management: The client should securely store the JWT, ideally in memory or a secure storage mechanism recommended for your platform, to prevent unauthorized access. The token should be attached to API requests in the Authorization header formatted as Bearer .
Forwarding Requests via BFF: When the client communicates with the Backend for Frontend service, the BFF should act as an intermediary. The BFF should verify the client’s JWT to ensure the request is from an authenticated user, typically by checking the signature and claims of the token (such as expiration).
Validating JWT at BFF: The BFF needs to validate the JWT’s claims, issuer (iss), subject (sub), audience (aud), and expiration (exp) to ensure it’s both legitimate and not expired. If your application involves specific permissions, the BFF might need to check additional claims that represent roles or scopes.
Token Forwarding or Issuance of a New Token: Depending on your architecture, the BFF may either directly forward the incoming JWT to the Resource Server, or might generate its own JWT to forward. If a new token is issued, it might encapsulate or supplement claims in the initial JWT to suit service-to-service communication demands.
JWT Validation at the Resource Server: Upon arrival, the Resource Server must also validate the JWT. This involves checking the token’s signature, claims, and ensuring it has sufficient permissions for the requested resource.
Response Propagation: Successful validation of the JWT at the Resource Server allows the request to be processed. The resource is then returned back to the client through the BFF.

Ensure that each service’s JWT validation logic is up-to-date with your security policies to prevent vulnerabilities. Also, make sure to handle token renewals and invalidations as necessary, possibly by incorporating a refresh token mechanism to limit the lifespan of JWTs and maintain session security. Regular audits and testing of the token flow can help ensure compliance and efficiency.