Cross-Origin Authentication

For most situations, Auth0 recommends that authentication transactions be handled via Universal Login. Doing so offers the easiest and most secure way to authenticate users. However, it is understood that some situations may require that authentication forms be directly embedded in an application. Cross-origin authentication provides a way to do this securely.

What is Cross-Origin Authentication?

When authentication requests are made from your application (via the Lock widget or a custom login form) to Auth0, the user's credentials are sent to a domain which differs from the one that serves your application. Collecting user credentials in an application served from one origin and then sending them to another origin can present certain security vulnerabilities, including the possibility of a phishing attack.

Auth0 provides a cross-origin authentication flow which makes use of third-party cookies. The use of third-party cookies allows Lock and Auth0's backend to perform the necessary checks to allow for secure authentication transactions across different origins. This helps to prevent phishing when creating a single sign-on experience with the Lock widget or a custom login form in your application and it also helps to create a secure login experience even if single sign-on is not the goal.

Cross-origin authentication is only necessary when authenticating against a directory using a username and password. Social IdPs and enterprise federation use a different mechanism, redirecting via standard protocols like OpenID Connect and SAML. Additionally, cross-origin authentication is only applicable to embedded login on the web (using Lock or auth0.js). Native applications using embedded login make use of the standard OAuth 2.0 token endpoint.

Security in deprecated library versions

Cross-origin authentication performed using deprecated versions of the Lock (< v11) and Auth0.js (< v9) libraries is unsafe, and the deprecated versions will be removed from service on July 16, 2018. For any applications which have yet to update and are still using embedded login from those deprecated libraries, a mitigation to the danger has been applied. All requests to the deprecated endpoints from those applications will be fingerprinted, to allow the Auth0 server to compare the request with previous ones and further mitigate risks. This measure does not prevent attacks, nor does it remove the need to migrate applications.

Limitations of Cross-Origin Authentication

Because cross-origin authentication is achieved using third-party cookies, disabling third-party cookies will make cross-origin authentication fail.

There are two approaches you can follow to remediate the issue:

  • Enable a Custom Domain on your tenant and host your web application in a domain that has the same top level domain as your Auth0 custom domain. Example: Host your application at and set your Auth0 custom domain as This way the cookies are no longer third-party (because both your Auth0 tenant and your application are using the same top level domain) and thus are not blocked by browsers.
  • Provide a Cross-Origin verification page that will make cross-origin authentication work in some browsers even with third-party cookies disabled (see the browser testing matrix below).

These issues are another reason why the more practical solution is to use Universal Login.

Configure Your Application for Cross-Origin Authentication

Configuring your application for cross-origin authentication is a process that requires a few steps:

  1. Ensure that the Allowed Web Origins field is set to the domain making the request. You can find this field in the Application Settings. Please note that the URLs specified for Allowed Web Origins cannot contain wildcards or relative paths after the domain.
  2. Ensure that your application is using Lock 11 or higher, or Auth0.js version 9 or higher.
  3. If you don't enable Custom Domains, you will need to author a page which uses auth0.js to act as a fallback for the cross-origin transaction. More information on setting up this page is provided below.

Create a Cross-Origin Verification Page

There are some cases when third party cookies will not be available. Certain browser versions do not support third party cookies and, if they do, there will be times that they will be disabled in a user's settings. You can use auth0.js in your application on a dedicated page to properly handle cases when third-party cookies are disabled. This page must be served over SSL.

Using crossOriginVerification as a fallback will only work if the browser is on the support matrix as Yes under "Third-Party Cookies Disabled". For some browsers, such as Chrome, Opera, and Safari, when third party cookies are disabled, cross-origin authentication will not work at all unless you enable Custom Domains.

Safari's configuration is labeled as "Prevent cross-site tracking" and uses Intelligent Tracking Prevention. Unfortunately, this also prevents 3rd party cookies from being useful in authentication scenarios. Here's an example of how it affects token renewal.

Provide a page in your application which instantiates WebAuth from auth0.js. Call crossOriginVerification immediately. The name of the page is at your discretion.

<!-- callback-cross-auth.html -->

  <script src=""></script>
  <script type="text/javascript">
    var auth0 = new auth0.WebAuth({
      clientID: 'YOUR_CLIENT_ID',
      domain: 'YOUR_AUTH0_DOMAIN'

When third party cookies are not available, auth0.js will render an iframe which will be used to call a different cross-origin verification flow.

Add the URL of this callback page to the Cross-Origin Verification Fallback field in your Application's settings in the Dashboard, under the Advanced > OAuth panel.

For production environments, verify that the Location URL for the page does not point to localhost.

See the cross-origin auth sample for more detail.

Error Codes and Descriptions

When Auth0.js v9 (and Lock v11) is used for embedded login, it employs the /co/authenticate endpoint, which has the following errors.

The error description is human readable. It should not be parsed by any code and it subject to change at any time.

Status Code Description
400 invalid_request Invalid request body. All and only of client_id, credential_type, username, otp, realm are required.
401 unauthorized_client Cross origin login not allowed.
400 unsupported_credential_type Unknown credential type parameter.
400 invalid_request Unknown realm non-existent-connection.
403 access_denied Invalid user credentials.
403 access_denied Wrong email or password.
403 access_denied Authentication error
403 blocked_user Blocked user
401 password_leaked This login attempt has been blocked because the password you're using was previously disclosed through a data breach (not in this application).
429 too_many_attempts Your account has been blocked after multiple consecutive login attempts. We’ve sent you an email with instructions on how to unblock it.
429 too_many_attempts We have detected suspicious login behavior and further attempts will be blocked. Please contact the administrator.

In addition, you can also get a generic 403 error without an error or error_description property. The response body would just include something similar to the following:

Origin is not allowed.

Browser Testing Matrix

This table lists which browsers can use cross-origin authentication when third-party cookies are disabled.

OS Browser Cross-Origin Authentication Supported
with Third-Party Cookies Disabled
Windows IE Yes
Windows Edge Yes
Windows Firefox No
Windows Chrome No
Windows Opera No
macOS Sierra Safari No
macOS Firefox No
macOS Chrome No
macOS Opera No
iOS (iPhone) Safari No
iOS (iPhone) Chrome No
iOS (iPhone) Firefox No
iOS (iPad) Safari No
iOS (iPad) Chrome No
Android Galaxy S7 Chrome No
Android Galaxy S7 Firefox No