Why does my deployed Firebase Cloud Function return a 403 error while it works locally?

It sounds like the issue is related to authentication and authorization. When testing locally with the emulator, Firebase might not enforce the full authentication rules that are enforced in production. Once deployed, your callable function is likely configured to only accept requests from authenticated users. This is why you’re seeing a 403 Forbidden error when the function is invoked on the cloud server.

Here are some steps to resolve the issue:

1. Verify Authentication in Your App

Ensure that your application is using Firebase Authentication so that every request to your callable function is coming from an authenticated user. For example, if you’re using the client SDK, make sure that you sign in users and pass along their authentication credentials when calling your function:

JAVASCRIPTCopied
// Example in JavaScript using Firebase Auth
firebase.auth().signInWithEmailAndPassword(email, password)
  .then((userCredential) => {
    // Get the ID token from the signed-in user
    return userCredential.user.getIdToken();
  })
  .then((idToken) => {
    const callable = firebase.functions().httpsCallable('myFunction');
    return callable({}, { headers: { Authorization: `Bearer ${idToken}` } });
  })
  .then((result) => {
    console.log(result.data);
  })
  .catch((error) => {
    console.error("Error calling function:", error);
  });

2. Check Function Security Rules

Your function might be configured to only allow users who are authenticated. Double-check your callable function code to ensure you’re explicitly verifying authentication. For example, in a Node.js Firebase function you can:

JAVASCRIPTCopied
exports.myFunction = functions.https.onCall((data, context) => {
  // Check if request is authenticated.
  if (!context.auth) {
    // Throwing an HttpsError so that the client gets the error details.
    throw new functions.https.HttpsError(
      'unauthenticated',
      'The request does not have valid authentication credentials for the operation.'
    );
  }
  // Continue with your Axios POST request to external API...
});

3. Testing on Production

Because the emulator may bypass full security rules or have different environments, always test on deployed functions to verify that your authentication is properly set up. If authentication is required, ensure that your clients are signed in before invoking the function.

4. External API Requirements

If your external API also implements its own authentication mechanisms, ensure that all required credentials (API keys, tokens, etc.) are correctly configured in your Firebase function when it makes the Axios POST request. Sometimes network environments or credentials might work locally but fail in the deployed environment if environment variables or secrets are not set up properly.

Summary

The 403 Forbidden error is likely because your deployed callable function is rejecting requests from non-authenticated users. Make sure to set up Firebase Authentication on your client side, verify the request in your callable function, and confirm that any external API credentials are also properly configured.