Authenticate with Firebase Using Email Link in Android

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

Set up your Android Studio project

  1. Add Firebase to your Android project.
  2. Add the dependencies for Firebase Authentication and Google Play services to your app-level build.gradle file:
    implementation 'com.google.firebase:firebase-auth:16.0.3'
    implementation 'com.google.android.gms:play-services-auth:15.0.1'
    

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.

To initiate the authentication flow, present the user with 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.

  1. 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).
    • 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.
    ActionCodeSettings actionCodeSettings =
            ActionCodeSettings.newBuilder()
                    // URL you want to redirect back to. The domain (www.example.com) for this
                    // URL must be whitelisted in the Firebase Console.
                    .setUrl("https://www.example.com/finishSignUp?cartId=1234")
                    // This must be true
                    .setHandleCodeInApp(true)
                    .setIOSBundleId("com.example.ios")
                    .setAndroidPackageName(
                            "com.example.android",
                            true, /* installIfNotAvailable */
                            "12"    /* minimumVersion */)
                    .build();
    

    To learn more on ActionCodeSettings, refer to the Passing State in Email Actions section.

  2. Ask the user for their email.

  3. 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.

    FirebaseAuth auth = FirebaseAuth.getInstance();
    auth.sendSignInLinkToEmail(email, actionCodeSettings)
            .addOnCompleteListener(new OnCompleteListener<Void>() {
                @Override
                public void onComplete(@NonNull Task<Void> task) {
                    if (task.isSuccessful()) {
                        Log.d(TAG, "Email sent.");
                    }
                }
            });
    

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.

Completing sign-in in an Android App

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.

Firebase Auth uses Firebase Dynamic Links when sending a link that is meant to be opened in a mobile application. In order to use this feature, Dynamic Links need to be configured in the Firebase Console.

  1. Enable Firebase Dynamic Links:

    1. In the Firebase console, open the Dynamic Links section.
    2. If you have not yet accepted the Dynamic Links terms and created a Dynamic Links domain, do so now.

      If you already created a Dynamic Links domain, take note of it. A Dynamic Links domain typically looks like the following example:

      example.page.link

      You will need this value when you configure your iOS or Android app to intercept the incoming link.

  2. Configuring Android applications:

    1. If you plan on handling these links from your Android application, the Android package name needs to be specified in the Firebase Console project settings. In addition, the SHA-1 and SHA-256 of the application certificate need to be provided.
    2. You will also need to configure the intent filter for the email action handler domain in your AndroidManifest.xml file. By default, the email action handler is hosted on a domain like the following example:
      APP_ID.firebaseapp.com/
    3. For more on this, refer to Receiving Android Dynamic Links instructions.

After you receive the link as described above, verify that it is meant for email link authentication and complete the sign in.

FirebaseAuth auth = FirebaseAuth.getInstance();
Intent intent = getIntent();
String emailLink = intent.getData().toString();

// Confirm the link is a sign-in with email link.
if (auth.isSignInWithEmailLink(emailLink)) {
    // Retrieve this from wherever you stored it
    String email = "someemail@domain.com";

    // The client SDK will parse the code from the link for you.
    auth.signInWithEmailLink(email, emailLink)
            .addOnCompleteListener(new OnCompleteListener<AuthResult>() {
                @Override
                public void onComplete(@NonNull Task<AuthResult> task) {
                    if (task.isSuccessful()) {
                        Log.d(TAG, "Successfully signed in with email link!");
                        AuthResult result = task.getResult();
                        // You can access the new user via result.getUser()
                        // Additional user info profile *not* available via:
                        // result.getAdditionalUserInfo().getProfile() == null
                        // You can check if the user is new or existing:
                        // result.getAdditionalUserInfo().isNewUser()
                    } else {
                        Log.e(TAG, "Error signing in with email link", task.getException());
                    }
                }
            });
}

To learn more on how to handle sign-in with email link in an iOS application, refer to the iOS guide.

To learn about how to handle sign-in with email link in a web application, refer to the Web guide.

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:

// Construct the email link credential from the current URL.
AuthCredential credential =
        EmailAuthProvider.getCredentialWithLink(email, emailLink);

// Link the credential to the current user.
auth.getCurrentUser().linkWithCredential(credential)
        .addOnCompleteListener(new OnCompleteListener<AuthResult>() {
            @Override
            public void onComplete(@NonNull Task<AuthResult> task) {
                if (task.isSuccessful()) {
                    Log.d(TAG, "Successfully linked emailLink credential!");
                    AuthResult result = task.getResult();
                    // You can access the new user via result.getUser()
                    // Additional user info profile *not* available via:
                    // result.getAdditionalUserInfo().getProfile() == null
                    // You can check if the user is new or existing:
                    // result.getAdditionalUserInfo().isNewUser()
                } else {
                    Log.e(TAG, "Error linking emailLink credential", task.getException());
                }
            }
        });

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

// Construct the email link credential from the current URL.
AuthCredential credential =
        EmailAuthProvider.getCredentialWithLink(email, emailLink);

// Re-authenticate the user with this credential.
auth.getCurrentUser().reauthenticateAndRetrieveData(credential)
        .addOnCompleteListener(new OnCompleteListener<AuthResult>() {
            @Override
            public void onComplete(@NonNull Task<AuthResult> task) {
                if (task.isSuccessful()) {
                    // User is now successfully reauthenticated
                } else {
                    Log.e(TAG, "Error reauthenticating", task.getException());
                }
            }
        });

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.

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:

auth.fetchSignInMethodsForEmail(email)
        .addOnCompleteListener(new OnCompleteListener<SignInMethodQueryResult>() {
            @Override
            public void onComplete(@NonNull Task<SignInMethodQueryResult> task) {
                if (task.isSuccessful()) {
                    SignInMethodQueryResult result = task.getResult();
                    List<String> signInMethods = result.getSignInMethods();
                    if (signInMethods.contains(EmailAuthProvider.EMAIL_PASSWORD_SIGN_IN_METHOD)) {
                        // User can sign in with email/password
                    } else if (signInMethods.contains(EmailAuthProvider.EMAIL_LINK_SIGN_IN_METHOD)) {
                        // User can sign in with email/link
                    }
                } else {
                    Log.e(TAG, "Error getting sign in methods for user", task.getException());
                }
            }
        });

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 signs in for the first time, a new user account is created and linked to the credentials—that is, the user name and password, phone number, or auth provider information—the user signed in with. This new 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 how the user signs in.

  • In your apps, you can get the user's basic profile information from the FirebaseUser 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:

FirebaseAuth.getInstance().signOut();

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