Authenticate with Firebase Using Email Links

Notice

This page is archived and might not reflect the latest version of the FlutterFire plugins. You can find the latest information on firebase.google.com:

https://firebase.google.com/docs/auth/flutter/email-link-auth

You can use Firebase Authentication to sign in a user by sending them an email containing a link, which they can click to sign in. In the process, the user's email address is also verified.

There are numerous benefits to signing in by email:

Low friction sign-up and sign-in.
Lower risk of password reuse across applications, which can undermine security of even well-selected passwords.
The ability to authenticate a user while also verifying that the user is the legitimate owner of an email address.
A user only needs an accessible email account to sign in. No ownership of a phone number or social media account is required.
A user can sign in securely without the need to provide (or remember) a password, which can be cumbersome on a mobile device.
An existing user who previously signed in with an email identifier (password or federated) can be upgraded to sign in with just the email. For example, a user who has forgotten their password can still sign in without needing to reset their password.

Before you begin#

If you haven't already, follow the steps in the Get started guide.
Enable Email Link sign-in for your Firebase project.
To sign in users by email link, you must first enable the Email provider and Email link sign-in method for your Firebase project:
1. In the Firebase console, open the Auth section.
2. On the Sign in method tab, enable the Email/Password provider. Note that email/password sign-in must be enabled to use email link sign-in.
3. In the same section, enable Email link (passwordless sign-in) sign-in method.
4. Click Save.

Send an authentication link to the user's email address#

To initiate the authentication flow, present an interface that prompts the user to provide their email address and then call sendSignInLinkToEmail() to request that Firebase send the authentication link to the user's email.

Construct the ActionCodeSettings object, which provides Firebase with instructions on how to construct the email link. Set the following fields:
- url: The deep link to embed and any additional state to be passed along. The link's domain has to be whitelisted in the Firebase Console list of authorized domains, which can be found by going to the Sign-in method tab (Authentication -> Sign-in method). The link will redirect the user to this URL if the app is not installed on their device and the app was not able to be installed.
- androidPackageName and IOSBundleId: The apps to use when the sign-in link is opened on an Android or iOS device. Learn more on how to configure Firebase Dynamic Links to open email action links via mobile apps.
- handleCodeInApp: Set to true. The sign-in operation has to always be completed in the app unlike other out of band email actions (password reset and email verifications). This is because, at the end of the flow, the user is expected to be signed in and their Auth state persisted within the app.
- dynamicLinkDomain: When multiple custom dynamic link domains are defined for a project, specify which one to use when the link is to be opened via a specified mobile app (for example, example.page.link). Otherwise the first domain is automatically selected.
var acs = ActionCodeSettings(
// URL you want to redirect back to. The domain (www.example.com) for this
// URL must be whitelisted in the Firebase Console.
url: 'https://www.example.com/finishSignUp?cartId=1234',
// This must be true
handleCodeInApp: true,
iOSBundleId: 'com.example.ios',
androidPackageName: 'com.example.android',
// installIfNotAvailable
androidInstallApp: true,
// minimumVersion
androidMinimumVersion: '12');
Ask the user for their email.
Send the authentication link to the user's email, and save the user's email in case the user completes the email sign-in on the same device.
var emailAuth = 'someemail@domain.com';
FirebaseAuth.instance.sendSignInLinkToEmail(
email: emailAuth, actionCodeSettings: acs)
.catchError((onError) => print('Error sending email verification $onError'))
.then((value) => print('Successfully sent email verification'));
});

Complete sign in with the email link#

Security concerns#

To prevent a sign-in link from being used to sign in as an unintended user or on an unintended device, Firebase Auth requires the user's email address to be provided when completing the sign-in flow. For sign-in to succeed, this email address must match the address to which the sign-in link was originally sent.

You can streamline this flow for users who open the sign-in link on the same device they request the link, by storing their email address locally - for instance using SharedPreferences - when you send the sign-in email. Then, use this address to complete the flow. Do not pass the user's email in the redirect URL parameters and re-use it as this may enable session injections.

After sign-in completion, any previous unverified mechanism of sign-in will be removed from the user and any existing sessions will be invalidated. For example, if someone previously created an unverified account with the same email and password, the user's password will be removed to prevent the impersonator who claimed ownership and created that unverified account from signing in again with the unverified email and password.

Also make sure you use an HTTPS URL in production to avoid your link being potentially intercepted by intermediary servers.

Verify email link and sign in#

Firebase Authentication uses Firebase Dynamic Links to send the email link to a mobile device. For sign-in completion via mobile application, the application has to be configured to detect the incoming application link, parse the underlying deep link and then complete the sign-in.

Set up your app to receive Dynamic Links on Flutter in the guide and ensure you have correctly integrated Dynamic Links for Android and Apple devices.
In your link handler, check if the link is meant for email link authentication and, if so, complete the sign-in process.
// Confirm the link is a sign-in with email link.
if (FirebaseAuth.instance.isSignInWithEmailLink(emailLink)) {
try {
// The client SDK will parse the code from the link for you.
final userCredential = await FirebaseAuth.instance
.signInWithEmailLink(email: emailAuth, emailLink: emailLink);

// You can access the new user via userCredential.user.
final emailAddress = userCredential.user?.email;

print('Successfully signed in with email link!');
} catch (error) {
print('Error signing in with email link.');
}
}

Linking/re-authentication with email link#

You can also link this method of authentication to an existing user. For example a user previously authenticated with another provider, such as a phone number, can add this method of sign-in to their existing account.

The difference would be in the second half of the operation:

final userCredential = await FirebaseAuth.instance
    .signInWithEmailLink(email: emailAuth, emailLink: emailLink);
try {
    await FirebaseAuth.instance.currentUser
        ?.linkWithCredential(userCredential.credential!);
} catch (error) {
    print("Error linking emailLink credential.");
}

This can also be used to re-authenticate an email link user before running a sensitive operation.

final userCredential = await FirebaseAuth.instance
    .signInWithEmailLink(email: emailAuth, emailLink: emailLink);
try {
    await FirebaseAuth.instance.currentUser
        ?.reauthenticateWithCredential(userCredential.credential!);
} catch (error) {
    print("Error reauthenticating credential.");
}

However, as the flow could end up on a different device where the original user was not logged in, this flow might not be completed. In that case, an error can be shown to the user to force them to open the link on the same device. Some state can be passed in the link to provide information on the type of operation and the user uid.

Differentiating email/password from email link#

In case you support both password and link-based sign in with email, to differentiate the method of sign in for a password/link user, use fetchSignInMethodsForEmail. This is useful for identifier-first flows where the user is first asked to provide their email and then presented with the method of sign-in:

try {
    final signInMethods =
        await FirebaseAuth.instance.fetchSignInMethodsForEmail(emailAuth);
    final userExists = signInMethods.isNotEmpty;
    final canSignInWithLink = signInMethods
        .contains(EmailAuthProvider.EMAIL_LINK_SIGN_IN_METHOD);
    final canSignInWithPassword = signInMethods
        .contains(EmailAuthProvider.EMAIL_PASSWORD_SIGN_IN_METHOD);
} on FirebaseAuthException catch (exception) {
    switch (exception.code) {
        case "invalid-email":
            print("Not a valid email address.");
            break;
        default:
            print("Unknown error.");
    }
}

As described above email/password and email/link are considered the same EmailAuthProvider (same PROVIDER_ID) with different methods of sign-in.

Next steps#

After a user creates a new account, this account is stored as part of your Firebase project, and can be used to identify a user across every app in your project, regardless of what sign-in method the user used.

In your apps, you can get the user's basic profile information from the User object. See Manage Users.

In your Firebase Realtime Database and Cloud Storage Security Rules, you can get the signed-in user's unique user ID from the auth variable, and use it to control what data a user can access.

You can allow users to sign in to your app using multiple authentication providers by linking auth provider credentials) to an existing user account.

To sign out a user, call signOut():

await FirebaseAuth.instance.signOut();