How do I resolve a Firebase ID token 'aud' claim mismatch in my Python backend?

Below is a list of possible causes and solutions for the audience (aud) claim mismatch error:

What the Error Means

The error indicates that when verifying the Firebase ID token on your backend, the token’s audience claim (aud) does not match the expected value from your service account. In your case, the expected audience is "bitpic" (your Firebase project identifier), but the token contains the value "1018060832031-v397dko55bp73mgr6r35hbvfkellsc7e.apps.googleusercontent.com" (the OAuth client ID).

Common Causes

• Mismatched Configuration:
  The frontend Firebase configuration might be using a different OAuth client ID from what your Firebase project (backend) expects. This can happen if you have multiple projects or if the OAuth client ID in your Firebase console does not match your admin SDK’s service account.

• Using the Wrong Token:
  Ensure that the token your frontend sends is indeed the Firebase ID token and not a raw Google OAuth token. The Admin SDK’s verify_id_token method expects an ID token issued by Firebase Authentication.

• Multiple Projects/Apps:
  Double-check that both your frontend and backend are configured for the same Firebase project.

How to Resolve

1. Check Your Firebase Configuration on the Frontend:
   Verify that the Firebase configuration object (e.g., in your web or mobile app) correctly references your project (project ID "bitpic") and that the OAuth client IDs are correctly set up in the Firebase console.

2. Review the Token Being Passed:
   Confirm that the token your frontend sends to your backend is the Firebase ID token, not a Google OAuth access token. The Firebase ID token’s payload will have the "aud" claim matching your Firebase project ID.

3. Validate the Service Account Used by the Admin SDK:
   Make sure that the service account you are using with firebase_admin.initialize_app(…) is indeed for the same Firebase project ("bitpic"). The printed project_id should match your project’s identifier.

4. Double-check Project Settings:
   Revisit your Firebase console settings for your Authentication providers. Ensure that the OAuth client ID registered here matches the one provided in your frontend configuration.

Example

If your frontend initialization looks like this:

JSCopied
const firebaseConfig = {
  apiKey: "YOUR_API_KEY",
  authDomain: "bitpic.firebaseapp.com",
  projectId: "bitpic",
  // other config properties
};

firebase.initializeApp(firebaseConfig);

Make sure that:

• The projectId matches the service account’s project_id.
• The OAuth client ID configured in the Firebase console is the one your app is actually using.

If the frontend is inadvertently using a different OAuth client ID (perhaps from another project or a direct Google API configuration), the token's aud claim will not match your expected Firebase project id.

Final Thoughts

The discrepancy you’re seeing suggests that the token being sent to your backend very likely comes from an OAuth flow that isn’t linked to your Firebase project, or it was misconfigured. Revisit both your client and server-side settings to ensure they target the same Firebase project, and verify that only Firebase ID tokens (with the correct audience) are passed to your backend for verification.

Following these steps should help you resolve the audience (aud) claim mismatch.