Extend Firebase Authentication with blocking Cloud Functions

Blocking functions let you execute custom code that modifies the result of a user registering or signing in to your app. For example, you can prevent a user from authenticating if they don't meet certain criteria, or update a user's information before returning it to your client app.

Before you begin

To use blocking functions you must upgrade your Firebase project to Firebase Authentication with Identity Platform. If you haven't already upgraded, do so first.

Understanding blocking functions

You can register blocking functions for two events:

  • beforeCreate: Triggers before a new user is saved to the Firebase Authentication database, and before a token is returned to your client app.

  • beforeSignIn: Triggers after a user's credentials are verified, but before Firebase Authentication returns an ID token to your client app. If your app uses multi-factor authentication, the function triggers after the user verifies their second factor. Note that creating a new user also triggers beforeSignIn, in addition to beforeCreate.

Keep the following in mind when using blocking functions:

  • Your function must respond within 7 seconds. After 7 seconds, Firebase Authentication returns an error, and the client operation fails.

  • HTTP response codes other than 200 are passed to your client apps. Ensure your client code handles any errors your function can return.

  • Functions apply to all users in your project, including any contained in a tenant. Firebase Authentication provides information about users to your function, including any tenants they belong to, so you can respond accordingly.

  • Linking another identity provider to an account re-triggers any registered beforeSignIn functions.

  • Anonymous and custom authentication do not trigger blocking functions.

Deploy and register a blocking function

To insert your custom code into the user authentication flows, deploy and register blocking functions. Once your blocking functions are deployed and registered, your custom code must complete successfully for authentication and user creation to succeed.

Deploy a blocking function

You deploy a blocking function the same way as you deploy any function. (see the Cloud Functions Getting started page for details). In summary:

  1. Write Cloud Functions that handle the beforeCreate event, the beforeSignIn event, or both.

    For example, to get started, you can add the following no-op functions to index.js:

    const functions = require('firebase-functions');
    exports.beforeCreate = functions.auth.user().beforeCreate((user, context) => {
      // TODO
    exports.beforeSignIn = functions.auth.user().beforeSignIn((user, context) => {
      // TODO

    The above examples have omitted the implementation of custom auth logic. See the following sections to learn how to implement your blocking functions and Common scenarios for specific examples.

  2. Deploy your functions using the Firebase CLI:

    firebase deploy --only functions

    You must re-deploy your functions each time you update them.

Register a blocking function

  1. Go to the Firebase Authentication Settings page in the Firebase console.

  2. Select the Blocking functions tab.

  3. Register your blocking function by selecting it from the dropdown menu under either Before account creation (beforeCreate) or Before sign in (beforeSignIn).

  4. Save your changes.

Getting user and context information

The beforeSignIn and beforeCreate events provide User and EventContext objects that contain information about the user signing in. Use these values in your code to determine whether to allow an operation to proceed.

For a list of properties available on the User object, see the UserRecord API reference.

The EventContext object contains the following properties:

Name Description Example
locale The application locale. You can set the locale using the client SDK, or by passing the locale header in the REST API. fr or sv-SE
ipAddress The IP address of the device the end user is registering or signing in from.
userAgent The user agent triggering the blocking function. Mozilla/5.0 (X11; Linux x86_64)
eventId The event's unique identifier. rWsyPtolplG2TBFoOkkgyg
eventType The event type. This provides information on the event name, such as beforeSignIn or beforeCreate, and the associated sign-in method used, like Google or email/password. providers/cloud.auth/eventTypes/user.beforeSignIn:password
authType Always USER. USER
resource The Firebase Authentication project or tenant. projects/project-id/tenants/tenant-id
timestamp The time the event was triggered, formatted as an RFC 3339 string. Tue, 23 Jul 2019 21:10:57 GMT
additionalUserInfo An object containing information about the user. AdditionalUserInfo
credential An object containing information about the user's credential. AuthCredential

Blocking registration or sign-in

To block a registration or sign-in attempt, throw an HttpsError in your function. For example:


throw new functions.auth.HttpsError('permission-denied');

The following table lists the errors you can raise, along with their default error message:

Name Code Message
invalid-argument 400 Client specified an invalid argument.
failed-precondition 400 Request can not be executed in the current system state.
out-of-range 400 Client specified an invalid range.
unauthenticated 401 Missing, invalid, or expired OAuth token.
permission-denied 403 Client does not have sufficient permission.
not-found 404 Specified resource is not found.
aborted 409 Concurrency conflict, such as a read-modify-write conflict.
already-exists 409 The resource that a client tried to create already exists.
resource-exhausted 429 Either out of resource quota or reaching rate limiting.
cancelled 499 Request cancelled by the client.
data-loss 500 Unrecoverable data loss or data corruption.
unknown 500 Unknown server error.
internal 500 Internal server error.
not-implemented 501 API method not implemented by the server.
unavailable 503 Service unavailable.
deadline-exceeded 504 Request deadline exceeded.

You can also specify a custom error message:


throw new functions.auth.HttpsError('permission-denied', 'Unauthorized request origin!');

The following example shows how to block users who are not within a specific domain from registering for your app:


exports.beforeCreate = functions.auth.user().beforeCreate((user, context) => {
  // (If the user is authenticating within a tenant context, the tenant ID can be determined from
  // user.tenantId or from context.resource, e.g. 'projects/project-id/tenant/tenant-id-1')

  // Only users of a specific domain can sign up.
  if (user.email.indexOf('@acme.com') === -1) {
    throw new functions.auth.HttpsError('invalid-argument', `Unauthorized email "${user.email}"`);

Regardless of whether you use a default or custom message, Cloud Functions wraps the error and returns it to the client as an internal error. For example:

throw new functions.auth.HttpsError('invalid-argument', `Unauthorized email user@evil.com}`);

Your app should catch the error, and handle it accordingly. For example:


// Blocking functions can also be triggered in a multi-tenant context before user creation.
// firebase.auth().tenantId = 'tenant-id-1';
firebase.auth().createUserWithEmailAndPassword('johndoe@example.com', 'password')
  .then((result) => {
  .then((idTokenResult) => {
  .catch((error) => {
    if (error.code !== 'auth/internal-error' && error.message.indexOf('Cloud Function') !== -1) {
      // Display error.
    } else {
      // Registration succeeds.

Modifying a user

Instead of blocking a registration or sign-in attempt, you can allow the operation to continue, but modify the User object that is saved to Firebase Authentication's database and returned to the client.

To modify a user, return an object from your event handler containing the fields to modify. You can modify the following fields:

  • displayName
  • disabled
  • emailVerified
  • photoUrl
  • customClaims
  • sessionClaims (beforeSignIn only)

With the exception of sessionClaims, all modified fields are saved to Firebase Authentication's database, which means they are included on the response token and persist between user sessions.

The following example shows how to set a default display name:


exports.beforeCreate = functions.auth.user().beforeCreate((user, context) => {
  return {
    // If no display name is provided, set it to "Guest".
    displayName: user.displayName || 'Guest';

If you register an event handler for both beforeCreate and beforeSignIn, note that beforeSignIn executes after beforeCreate. User fields updated in beforeCreate are visible in beforeSignIn. If you set a field other than sessionClaims in both event handlers, the value set in beforeSignIn overwrites the value set in beforeCreate. For sessionClaims only, they are propagated to the current session's token claims, but are not persisted or stored in the database.

For example, if any sessionClaims are set, beforeSignIn will return them with any beforeCreate claims, and they will be merged. When they're merged, if a sessionClaims key matches a key in customClaims, the matching customClaims will be overwritten in the token claims by the sessionClaims key. However, the overwitten customClaims key will still be persisted in the database for future requests.

Supported OAuth credentials and data

You can pass OAuth credentials and data to blocking functions from various identity providers. The following table shows what credentials and data are supported for each identity provider:

Identity Provider ID Token Access Token Expiration Time Token Secret Refresh Token Sign-in Claims
Google Yes Yes Yes No Yes No
Facebook No Yes Yes No No No
Twitter No Yes No Yes No No
GitHub No Yes No No No No
Microsoft Yes Yes Yes No Yes No
LinkedIn No Yes Yes No No No
Yahoo Yes Yes Yes No Yes No
Apple Yes Yes Yes No Yes No
SAML No No No No No Yes
OIDC Yes Yes Yes No Yes Yes

Refresh tokens

To use a refresh token in a blocking function, you must first select the checkbox on the Blocking functions page of the Firebase console.

Refresh tokens will not be returned by any identity providers when signing in directly with an OAuth credential, such as an ID token or access token. In this situation, the same client-side OAuth credential will be passed to the blocking function.

The following sections describe each identity provider types and their supported credentials and data.

Generic OIDC providers

When a user signs in with a generic OIDC provider, the following credentials will be passed:

  • ID token: Provided if the id_token flow is selected.
  • Access token: Provided if the code flow is selected. Note that the code flow is only currently supported via the REST API.
  • Refresh token: Provided if the offline_access scope is selected.


const provider = new firebase.auth.OAuthProvider('oidc.my-provider');


When a user signs in with Google, the following credentials will be passed:

  • ID token
  • Access token
  • Refresh token: Only provided if the following custom parameters are requested:
    • access_type=offline
    • prompt=consent, if the user previously consented and no new scope was requested


const provider = new firebase.auth.GoogleAuthProvider();
  'access_type': 'offline',
  'prompt': 'consent'

Learn more about Google refresh tokens.


When a user signs in with Facebook, the following credential will be passed:

  • Access token: An access token is returned that can be exchanged for another access token. Learn more about the different types of access tokens supported by Facebook and how you can exchange them for long-lived tokens.


When a user signs in with GitHub, the following credential will be passed:

  • Access token: Does not expire unless revoked.


When a user signs in with Microsoft, the following credentials will be passed:

  • ID token
  • Access token
  • Refresh token: Passed to the blocking function if the offline_access scope is selected.


const provider = new firebase.auth.OAuthProvider('microsoft.com');


When a user signs in with Yahoo, the following credentials will be passed without any custom parameters or scopes:

  • ID token
  • Access token
  • Refresh token


When a user signs in with LinkedIn, the following credential will be passed:

  • Access token


When a user signs in with Apple, the following credentials will be passed without any custom parameters or scopes:

  • ID token
  • Access token
  • Refresh token

Common scenarios

The following examples demonstrate some common use cases for blocking functions:

Only allowing registration from a specific domain

The following example shows how to prevent users who aren't part of the example.com domain from registering with your app:


exports.beforeCreate = functions.auth.user().beforeCreate((user, context) => {
  if (!user.email || user.email.indexOf('@example.com') === -1) {
    throw new functions.auth.HttpsError(
      'invalid-argument', `Unauthorized email "${user.email}"`);

Blocking users with unverified emails from registering

The following example shows how to prevent users with unverified emails from registering with your app:


exports.beforeCreate = functions.auth.user().beforeCreate((user, context) => {
  if (user.email && !user.emailVerified) {
    throw new functions.auth.HttpsError(
      'invalid-argument', `Unverified email "${user.email}"`);

Requiring email verification on registration

The following example shows how to require a user to verify their email after registering:


exports.beforeCreate = functions.auth.user().beforeCreate((user, context) => {
  const locale = context.locale;
  if (user.email && !user.emailVerified) {
    // Send custom email verification on sign-up.
    return admin.auth().generateEmailVerificationLink(user.email).then((link) => {
      return sendCustomVerificationEmail(user.email, link, locale);

exports.beforeSignIn = functions.auth.user().beforeSignIn((user, context) => {
 if (user.email && !user.emailVerified) {
   throw new functions.auth.HttpsError(
     'invalid-argument', `"${user.email}" needs to be verified before access is granted.`);

Treating certain identity provider emails as verified

The following example shows how to treat user emails from certain identity providers as verified:


exports.beforeCreate = functions.auth.user().beforeCreate((user, context) => {
  if (user.email && !user.emailVerified && context.eventType.indexOf(':facebook.com') !== -1) {
    return {
      emailVerified: true,

Blocking sign-in from certain IP addresses

The following example how block sign-in from certain IP address ranges:


exports.beforeSignIn = functions.auth.user().beforeSignIn((user, context) => {
  if (isSuspiciousIpAddress(context.ipAddress)) {
    throw new functions.auth.HttpsError(
      'permission-denied', 'Unauthorized access!');

Setting custom and session claims

The following example shows how to set custom and session claims:


exports.beforeCreate = functions.auth.user().beforeCreate((user, context) => {
  if (context.credential &&
      context.credential.providerId === 'saml.my-provider-id') {
    return {
      // Employee ID does not change so save in persistent claims (stored in
      // Auth DB).
      customClaims: {
        eid: context.credential.claims.employeeid,
      // Copy role and groups to token claims. These will not be persisted.
      sessionClaims: {
        role: context.credential.claims.role,
        groups: context.credential.claims.groups,

Tracking IP addresses to monitor suspicious activity

You can prevent token theft by tracking the IP address a user signs in from, and comparing it to the IP address on subsequent requests. If the request appears suspicious — for example, the IPs are from from different geographical regions — you can ask the user to sign in again.

  1. Use session claims to track the IP address the user signs in with:


    exports.beforeSignIn = functions.auth.user().beforeSignIn((user, context) => {
      return {
        sessionClaims: {
          signInIpAddress: context.ipAddress,
  2. When a user attempts to access resources that require authentication with Firebase Authentication, compare the IP address in the request with the IP used to sign in:


    app.post('/getRestrictedData', (req, res) => {
      // Get the ID token passed.
      const idToken = req.body.idToken;
      // Verify the ID token, check if revoked and decode its payload.
      admin.auth().verifyIdToken(idToken, true).then((claims) => {
        // Get request IP address
        const requestIpAddress = req.connection.remoteAddress;
        // Get sign-in IP address.
        const signInIpAddress = claims.signInIpAddress;
        // Check if the request IP address origin is suspicious relative to
        // the session IP addresses. The current request timestamp and the
        // auth_time of the ID token can provide additional signals of abuse,
        // especially if the IP address suddenly changed. If there was a sudden
        // geographical change in a short period of time, then it will give
        // stronger signals of possible abuse.
        if (!isSuspiciousIpAddressChange(signInIpAddress, requestIpAddress)) {
          // Suspicious IP address change. Require re-authentication.
          // You can also revoke all user sessions by calling:
          // admin.auth().revokeRefreshTokens(claims.sub).
          res.status(401).send({error: 'Unauthorized access. Please login again!'});
        } else {
          // Access is valid. Try to return data.
          getData(claims).then(data => {
          }, error => {
            res.status(500).send({ error: 'Server error!' })

Screening user photos

The following example shows how to sanitize users' profile photos:


exports.beforeCreate = functions.auth.user().beforeCreate((user, context) => {
  if (user.photoURL) {
    return isPhotoAppropriate(user.photoURL)
      .then((status) => {
        if (!status) {
          // Sanitize inappropriate photos by replacing them with guest photos.
          // Users could also be blocked from sign-up, disabled, etc.
          return {
            photoUrl: PLACEHOLDER_GUEST_PHOTO_URL,

To learn more about how to detect and sanitize images, see the Cloud Vision documentation.

Accessing a user's identity provider OAuth credentials

The following example demonstrates how to obtain a refresh token for a user that signed in with Google, and use it to call the Google Calendar APIs. The refresh token is stored for offline access.


const {OAuth2Client} = require('google-auth-library');
const {google} = require('googleapis');
// ...
// Initialize Google OAuth client.
const keys = require('./oauth2.keys.json');
const oAuth2Client = new OAuth2Client(

exports.beforeCreate = functions.auth.user().beforeCreate((user, context) => {
  if (context.credential &&
      context.credential.providerId === 'google.com') {
    // Store the refresh token for later offline use.
    // These will only be returned if refresh tokens credentials are included
    // (enabled by Cloud console).
    return saveUserRefreshToken(
      .then(() => {
        // Blocking the function is not required. The function can resolve while
        // this operation continues to run in the background.
        return new Promise((resolve, reject) => {
          // For this operation to succeed, the appropriate OAuth scope should be requested
          // on sign in with Google, client-side. In this case:
          // https://www.googleapis.com/auth/calendar
          // You can check granted_scopes from within:
          // context.additionalUserInfo.profile.granted_scopes (space joined list of scopes).

          // Set access token/refresh token.
            access_token: context.credential.accessToken,
            refresh_token: context.credential.refreshToken,
          const calendar = google.calendar('v3');
          // Setup Onboarding event on user's calendar.
          const event = {/** ... */};
            auth: oauth2client,
            calendarId: 'primary',
            resource: event,
          }, (err, event) => {
            // Do not fail. This is a best effort approach.