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:

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) {
  final String error = switch (e.code) {
    'invalid-email' => 'The email address is not valid.',
    // Email enumeration protection (default since Sep 2023) reports
    // 'invalid-credential' instead of 'user-not-found' / 'wrong-password';
    // older native SDKs report 'invalid-login-credentials'. Handle all four so
    // the app works with the setting on or off.
    'invalid-credential' ||
    'invalid-login-credentials' ||
    'user-not-found' ||
    'wrong-password' =>
      'The email address or password is incorrect.',
    'user-disabled' => 'This account has been disabled.',
    'too-many-requests' ||
    'network-request-failed' =>
      'Something went wrong. Try again later.',
    // Always keep a default branch — codes differ by platform and SDK version
    // (for example Windows reports 'unknown-error' for a wrong password).
    _ => 'Sign-in failed (${e.code}).',
  };
  print(error);
}

• 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"):

```dart
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.

```dart
// 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.
• Branch on e.code, never on e.message — messages come from the native SDKs, differ between platforms, and change between SDK versions.
• Codes differ between platforms for the same situation (e.g., Windows uses unknown-error, no-signed-in-user, account-exists-with-different-credentials). Check the error code reference when targeting several platforms.
• Catch FirebaseAuthMultiFactorException by type, before FirebaseAuthException, and complete sign-in with e.resolver; its code differs by platform (second-factor-required vs multi-factor-auth-required).
• signInWithRedirect, getRedirectResult and reauthenticateWithPopup are the exception — they throw a plain FirebaseAuthException carrying the multi-factor code, without a resolver. Branch on the code in those flows, or enrolled users are stranded on a generic error with no way to complete the second factor.
• On Android and Apple, verifyPhoneNumber does not throw — errors arrive in the verificationFailed callback.
• Handle account-exists-with-different-credential by signing the user in with the provider their account already uses, then calling linkWithCredential(e.credential). With email enumeration protection enabled, Firebase does not reveal which provider that is, so let the user choose.
• 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.

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.

References

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