Open Redirect via "next" parameter in third-party authentication

How it was found

Hi,

It is currently possible to execute an open redirection attack via the next parameter with the inclusion of a triple-slash prefix.

Proof of Concept

Redirect URL

https://demo.weblate.org/accounts/login/github/?next=///google.com

After authenticating, the user will be immediately redirected to the attacker-specified target. I believe this affects all third-party authentication providers on the Weblate platform.

Please let me know if you require any additional details regarding this vulnerability.

Thanks!