Modern applications cannot rely solely on frontend authentication.

In real-world production systems - especially SaaS, fintech, and enterprise platforms - authentication must be verified and owned by the backend, not blindly trusted from the client. This is particularly important in mobile applications where client-side logic can be tampered with and tokens can be intercepted if not handled correctly.

If you’re building a Flutter-based product at scale, these concerns typically surface alongside broader questions around Flutter application security and backend ownership, which we’ve explored in detail in our guide on Flutter app security best practices.

While FlutterFlow provides fast UI development and Firebase-native authentication, serious production systems quickly hit limitations. This is especially true for teams building production-grade mobile products where authentication, session management, and backend verification must scale securely across platforms. These challenges typically surface once an app moves beyond prototyping into real-world mobile application development, where backend ownership and security can no longer be treated as optional.

Why FlutterFlow’s Built-In Auth Is Not Enough

FlutterFlow’s default Google and Apple sign-in flows are tightly coupled to Firebase.

This works well for prototypes, but it falls short when you need:

• A custom backend
• Server-side token verification
• Platform-agnostic authentication logic
• Independence from Firebase as the source of truth

These limitations are similar to what teams face when moving from no-code or low-code tooling to custom software architectures designed for long-term scale, especially when multiple clients or platforms must share a single identity system.

Core Limitations

• OAuth tokens are not reliably accessible
• Tokens cannot be forwarded cleanly to REST APIs
• Backend cannot independently verify identity
• Firebase implicitly becomes the authority

For production-grade systems, this creates unnecessary risk.

Backend-First Authentication: The Correct Mental Model

In secure systems:

Clients authenticate. Backends verify. Backends issue sessions.

Your backend - not FlutterFlow, not Firebase - must remain the single source of truth for identity, roles, and permissions.

This same principle applies across enterprise-grade application development, where backend-controlled authentication enables consistent security policies across mobile, web, and future clients.

Architecture Overview

End-to-End Flow

1. User signs in with Google or Apple
2. FlutterFlow Custom Action captures OAuth tokens
3. Tokens are sent to your REST API
4. Backend verifies tokens with Google or Apple
5. Backend creates or updates the user record
6. Backend issues its own JWT or session token
7. Client stores backend token and continues

This approach mirrors how authentication is handled in enterprise application development and regulated environments where client trust is explicitly limited.

Custom Action: Google Sign-In in FlutterFlow

Google Sign-In is relatively straightforward, but FlutterFlow’s built-in abstraction hides critical token details that your backend needs.

By implementing Google Sign-In via a Custom Action, you regain full control over:

• OAuth token handling
• Backend verification
• Session ownership

This pattern is especially useful when building Flutter-based products that integrate with existing backend systems or third-party services.

Google Sign-In Custom Action Code

import 'package:firebase_auth/firebase_auth.dart';

import 'package:google_sign_in/google_sign_in.dart';

Future<GoogleUserResponseStruct?> signInWithGoogle() async {

  try {

    final GoogleSignInAccount? googleUser = await GoogleSignIn().signIn();

    if (googleUser == null) {

      return null;

    }

    final GoogleSignInAuthentication googleAuth =

        await googleUser.authentication;

    final accessToken = googleAuth.accessToken;

    final idToken = googleAuth.idToken;

    final credential = GoogleAuthProvider.credential(

      accessToken: accessToken,

      idToken: idToken,

    );

    UserCredential userCredential =

        await FirebaseAuth.instance.signInWithCredential(credential);

    User? user = userCredential.user;

    return GoogleUserResponseStruct(

      accessToken: accessToken ?? '',

      idToken: idToken ?? '',

      uid: user?.uid ?? '',

      name: user?.displayName ?? googleUser.displayName ?? '',

      email: user?.email ?? googleUser.email,

      photoUrl: user?.photoURL ?? googleUser.photoUrl ?? '',

    );

  } catch (e) {

    print("🍋 Google sign-in error: $e");

    return null;

  }

}

Firebase here is optional. Even when used, it should never replace backend verification - a principle that also applies when designing secure Flutter authentication flows at scale.

Custom Action: Apple Sign-In in FlutterFlow

Apple Sign-In introduces stricter privacy and data retention constraints, making backend-first handling even more critical.

If you are building for iOS, Apple Sign-In is mandatory in many cases, and mistakes here often surface only after apps are live in production.

Apple-Specific Constraints You Must Handle

• Email and name are returned only on the first sign-in
• Data must be stored immediately
• Tokens must be verified using Apple public keys
• Authorization codes must be exchanged server-side

These requirements are similar to other regulated identity flows, such as those used in fintech or enterprise platforms where identity data cannot be re-requested later.

Apple Sign-In Custom Action Code

Future<AppleUserResponseStruct?> signInWithApple() async {
  try {
    debugPrint("Launching Apple Sign-In...");
    final appleCredential =
        await SignInWithApple.getAppleIDCredential(
      scopes: [
        AppleIDAuthorizationScopes.email,
        AppleIDAuthorizationScopes.fullName,
      ],
    ).timeout(
      const Duration(seconds: 20),
      onTimeout: () {
        throw Exception("Apple Sign-In timed out.");
      },
    );
    return AppleUserResponseStruct(
      idToken: appleCredential.identityToken ?? '',
      accessToken: appleCredential.authorizationCode,
      uid: appleCredential.userIdentifier ?? '',
      name:
          "${appleCredential.givenName ?? ''} ${appleCredential.familyName ?? ''}"
              .trim(),
      email: appleCredential.email ?? '',
    );
  } catch (e, st) {
    debugPrint("Apple sign-in error: $e");
    debugPrintStack(stackTrace: st);
    return null;
  }
}

This design ensures your backend - not the client - controls identity persistence, which is critical when scaling Flutter apps into multi-tenant or enterprise systems.

FlutterFlow Action Setup (Clean & Maintainable)

FlutterFlow’s visual action builder works best when logic remains simple.

The recommended pattern:

• Custom Action → Validate Response → Backend API → Navigate

This avoids fragmented auth logic and aligns with backend-driven application architecture best practices.

Backend Responsibilities (Non-Negotiable)

Your backend must treat all incoming tokens as untrusted input until verified.

At minimum, it must:

• Verify Google ID tokens
• Verify Apple identity tokens
• Exchange Apple authorization codes
• Normalize user identity
• Issue backend-controlled sessions
• Attach roles, flags, and permissions

This same backend-first approach is foundational in custom software development for startups and enterprises, where long-term maintainability matters more than speed alone.

Why This Pattern Is Production-Ready

This architecture:

• Removes Firebase lock-in
• Scales across platforms
• Centralizes security decisions
• Keeps FlutterFlow logic simple
• Enables future SSO providers

It’s the same model used in mature SaaS products and enterprise ecosystems.

Common Mistakes to Avoid

• Trusting Firebase UID without backend verification
• Losing Apple email/name after first login
• Accepting tokens without validation
• Mixing FlutterFlow auth with parallel user models
• Ignoring cancelled or null sign-in flows

Most of these mistakes only surface after launch - when fixing them is expensive.

Final Thoughts

FlutterFlow is excellent for accelerating UI and feature delivery, but authentication must remain backend-owned.

By combining:

• FlutterFlow Custom Actions
• Native OAuth flows
• Backend token verification
• Your own session layer

You create an authentication system that is:

• Secure
• Scalable
• Platform-agnostic
• Production-ready

This pattern is a strong default for any FlutterFlow application that integrates with a custom backend or enterprise architecture.