Cross-Origin Authentication

Auth0 strongly recommends that authentication transactions be handled via Universal Login because it is the easiest and most secure way to authenticate users. To learn more, read Centralized Universal Login vs. Embedded Login. There are a limited number of situations that may require that authentication forms be directly embedded in an application. Although not recommended, cross-origin authentication provides a way to do this.

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 that 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 that uses 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 SSO is not the goal.

Cross-origin authentication is not recommended and 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.


Because cross-origin authentication is achieved using third-party cookies, disabling third-party cookies will make cross-origin authentication fail. Some browsers, such as the newest version of Firefox, disable third-party cookies by default, meaning that cross-origin authentication will not work for users on Firefox. The only way to make embedded login work for Firefox users is to use a custom domain, as described below.

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. For example, you host an 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.

  • Create and link a cross-origin verification page that will make cross-origin authentication work in a limited number of browsers even with third-party cookies disabled.

Learn more