Firebase Authentication Skill

This skill defines how to correctly use Firebase Authentication in Flutter applications.

When to Use

Use this skill when:

• Setting up Firebase Authentication in a Flutter project.
• Listening to authentication state changes.
• Implementing email/password, phone number, or social sign-in.
• Managing user profiles, account linking, or MFA.
• Handling authentication errors (including iOS recaptcha-sdk-not-linked for phone auth).
• Applying security best practices for auth flows.

---

1. Setup and Configuration

flutter pub add firebase_auth

import 'package:firebase_auth/firebase_auth.dart';

• Enable desired authentication providers in the Firebase console before using them.
• Initialize Firebase before using any Firebase Authentication features.

Local emulator for testing:

Future<void> main() async {
  WidgetsFlutterBinding.ensureInitialized();
  await Firebase.initializeApp();
  await FirebaseAuth.instance.useAuthEmulator('localhost', 9099);
  // ...
}

---

2. Authentication State Management

Use the appropriate stream based on what you need to observe:

Stream	Fires when
authStateChanges()	User signs in or out
idTokenChanges()	ID token changes (including custom claims)
userChanges()	User data changes (e.g., profile updates)

FirebaseAuth.instance
  .authStateChanges()
  .listen((User? user) {
    if (user == null) {
      print('User is currently signed out!');
    } else {
      print('User is signed in!');
    }
  });

• Listen to these streams immediately when the app starts to handle the initial auth state.
• Custom claims are only available after sign-in, re-authentication, token expiration, or manual token refresh.

---

3. Email and Password Authentication

Create a new account:

try {
  final credential = await FirebaseAuth.instance.createUserWithEmailAndPassword(
    email: emailAddress,
    password: password,
  );
} on FirebaseAuthException catch (e) {
  if (e.code == 'weak-password') {
    print('The password provided is too weak.');
  } else if (e.code == 'email-already-in-use') {
    print('The account already exists for that email.');
  }
} catch (e) {
  print(e);
}

Sign in:

try {
  final credential = await FirebaseAuth.instance.signInWithEmailAndPassword(
    email: emailAddress,
    password: password,
  );
} on FirebaseAuthException catch (e) {
  if (e.code == 'invalid-credential') {
    // Email enumeration protection enabled (default since Sep 2023):
    // replaces 'user-not-found' and 'wrong-password'.
    print('Invalid email or password.');
  } else if (e.code == 'user-not-found') {
    print('No user found for that email.');
  } else if (e.code == 'wrong-password') {
    print('Wrong password provided for that user.');
  }
}

• Verify the user's email address after account creation.
• Firebase rate-limits new email/password sign-ups from the same IP to protect against abuse.
• On iOS/macOS, authentication state persists between app re-installs via the system keychain.
• Since September 2023, Firebase enables email enumeration protection by default on new projects, replacing user-not-found and wrong-password with invalid-credential. Manage this in the Firebase console under Authentication > Settings.
• When email enumeration protection is enabled, sendPasswordResetEmail() may complete without an error even if the email is not registered. Treat this as expected behavior and do not use password-reset responses to infer whether an email exists.

Share authentication state between Apple apps:

On Apple platforms, share auth state between apps in the same developer account by storing it in a shared Keychain access group. Enable the Keychain Sharing capability for each app with the same access group, then configure Firebase Auth with the fully qualified access group:

await FirebaseAuth.instance.setSettings(
  userAccessGroup: 'TEAMID.com.example.group1',
);

Switching from the default Keychain to a shared access group signs out the existing user unless migrated. Pass migrateCurrentUser: true (requires a non-null userAccessGroup) to preserve the current session, including an anonymous one — safe to call at app startup before any other Auth call, since it reads the existing session straight from the Keychain before Auth restores it. Migration can overwrite a user already stored in the destination access group.

---

4. Social Authentication

Google Sign-In (native platforms):

Future<UserCredential> signInWithGoogle() async {
  final GoogleSignInAccount? googleUser = await GoogleSignIn.instance.authenticate();
  final GoogleSignInAuthentication googleAuth = googleUser.authentication;
  final credential = GoogleAuthProvider.credential(idToken: googleAuth.idToken);
  return await FirebaseAuth.instance.signInWithCredential(credential);
}

Google Sign-In (web):

Future<UserCredential> signInWithGoogle() async {
  GoogleAuthProvider googleProvider = GoogleAuthProvider();
  googleProvider.addScope('https://www.googleapis.com/auth/contacts.readonly');
  googleProvider.setCustomParameters({'login_hint': 'user@example.com'});
  return await FirebaseAuth.instance.signInWithPopup(googleProvider);
}

• Configure platform-specific settings for each provider (e.g., SHA1 key for Google Sign-In on Android).
• If a user signs in with a social provider after registering with the same email manually, Firebase's trusted provider concept will automatically change their authentication provider.
• On Android, signInWithProvider opens a Chrome Custom Tab. If AndroidManifest.xml contains android:taskAffinity="" (Flutter's default), the tab closes when the user switches apps (e.g., to use a password manager), causing a web-context-already-presented error. Remove android:taskAffinity="" to fix this.
• When signing in with Apple, add the email and name scopes to present the full first-time sign-in UI (including "Share/Hide email"):

  final appleProvider = AppleAuthProvider();
  appleProvider.addScope('email');
  appleProvider.addScope('name');
  

• To revoke Apple auth tokens after sign-in, use the appropriate API per platform:
  • Apple platforms (iOS/macOS/web): use revokeTokenWithAuthorizationCode() with the authorization code from userCredential.additionalUserInfo?.authorizationCode.
  • Android: use revokeAccessToken() with the access token from userCredential.credential?.accessToken.

  // Apple platforms (iOS/macOS/web)
  final authCode = userCredential.additionalUserInfo?.authorizationCode;
  if (authCode != null) {
    await FirebaseAuth.instance.revokeTokenWithAuthorizationCode(authCode);
  }
  
  // Android
  final accessToken = userCredential.credential?.accessToken;
  if (accessToken != null) {
    await FirebaseAuth.instance.revokeAccessToken(accessToken);
  }
  

---

5. Phone Number Authentication

Before using phone authentication, ensure platform-specific prerequisites are met:

• Android: SHA-1 hashes must be configured in the Firebase console and Google Play Integrity API enabled.
• iOS: APNs authentication key must be configured with FCM and background modes for remote notifications enabled.
• Web: Add your application's domain to the Firebase console under OAuth redirect domains.

Phone number sign-in is only supported on real devices and the web. Testing on device emulators is not supported.

iOS: recaptcha-sdk-not-linked error

On iOS, verifyPhoneNumber can throw FirebaseAuthException with code recaptcha-sdk-not-linked when Identity Platform expects reCAPTCHA Enterprise but the native SDK is not linked. This cannot be resolved from Dart — fix it at the native iOS or GCP level:

• Recommended: Link the reCAPTCHA Enterprise iOS SDK in Xcode following Google's guide.
• Alternative: Disable reCAPTCHA SMS defense via the Identity Toolkit projects.updateConfig REST API (set recaptchaConfig.phoneEnforcementState to OFF and recaptchaConfig.useSmsTollFraudProtection to false). See the official steps. This reduces fraud protection — prefer linking the SDK.
• If the SDK uses a Safari view controller-hosted challenge, handle the return URL using uni_links/app_links or application:openURL: in the iOS runner.

---

6. Error Handling

• Always use try-catch with FirebaseAuthException.
• Check e.code to identify specific error types.
• Handle account-exists-with-different-credential by fetching sign-in methods for the email and guiding users through the correct flow.
• Handle too-many-requests with retry logic or user feedback.
• Handle operation-not-allowed by ensuring the provider is enabled in the Firebase console.
• On iOS, recaptcha-sdk-not-linked during verifyPhoneNumber is raised by the native Firebase iOS Auth SDK and requires native setup or GCP configuration changes — it cannot be fixed from Dart code alone.

---

7. User Management

// Update profile
await FirebaseAuth.instance.currentUser?.updateProfile(
  displayName: "Jane Q. User",
  photoURL: "https://example.com/jane-q-user/profile.jpg",
);

// Update email (sends verification to new address first)
await user?.verifyBeforeUpdateEmail("newemail@example.com");

• Use verifyBeforeUpdateEmail() — not updateEmail() — to change a user's email. The email only updates after the user verifies it.
• Store only essential info in the auth profile; use a database for additional user data.
• Use linkWithCredential() to connect multiple auth providers to a single account.
• Verify the user's identity before linking new credentials.
• Use fetchSignInMethodsForEmail() when handling account linking.

---

8. Security Best Practices

• Never store sensitive authentication credentials in client-side code.
• Monitor auth state changes for proper session management.
• Validate user input before submitting authentication requests to prevent injection attacks.
• Call FirebaseAuth.instance.signOut() when users exit the app.
• For sensitive operations, re-authenticate users with reauthenticateWithCredential().
• Enforce strong password policies for email/password auth.
• In Realtime Database and Cloud Storage Security Rules, use the auth variable to get the signed-in user's UID for access control.
• Use multi-factor authentication for sensitive applications.

---

9. Multi-Factor Authentication

Security warning: Avoid SMS-based MFA. SMS is insecure and easy to compromise or spoof.

Platform limitation: Windows does not support MFA. MFA with multiple tenants is not supported on Flutter.

• Enable at least one MFA-compatible provider before implementing MFA.

---

10. Email Link Authentication

Important: Firebase Dynamic Links is deprecated for email link authentication. Firebase Hosting is now used to send sign-in links.

• Set handleCodeInApp: true in ActionCodeSettings — sign-in must always be completed in the app.
• Store the user's email locally (e.g., SharedPreferences) when sending the sign-in link.
• Never pass the user's email in redirect URL parameters — this enables session injection attacks.
• Use HTTPS URLs in production to prevent link interception.
• Configure the app to detect incoming links and parse the underlying deep link for sign-in completion.

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References

• Firebase Authentication Flutter documentation
• Email/password authentication
• Federated identity & social sign-in
• Multi-factor authentication