Hi @sabina96clerk, and thank you for bringing this subject back to light so we can provide a solution!
The root cause of the “No verifier returned from client” error is that the method uses auth.login() with a username and password and relies on cross-origin authentication using third-party cookies.
Modern browsers, particularly Firefox (with Enhanced Tracking Protection) and Chrome Incognito mode, block third-party cookies by default for privacy reasons. This breaks the communication channel that auth0.js uses to complete the login, leading to the error you’re seeing. The “Cross-Origin Verification Fallback” is a legacy mechanism that also struggles with these modern privacy restrictions and cannot fix this underlying issue.
To fix this reliably across all browsers, you must switch from the cross-origin auth.login() method to the redirect-based Authorization Code Flow. This flow does not depend on third-party cookies and is the current best practice.
The change is straightforward: instead of logging the user in behind the scenes, you will redirect them to the Auth0 Universal Login page to enter their credentials. So you need to replace the webAuth.login() call with webAuth.authorize(). This method will handle redirecting the user to your Auth0 login page.
Please take a look at our docs for an example.
I hope this helps you!
Teodor.