Home assistant is an open source home automation. The Home Assistant login page allows users to use their local Home Assistant credentials and log in to another website that specifies the `redirect_uri` and `client_id` parameters. Although the `redirect_uri` validation typically ensures that it matches the `client_id` and the scheme represents either `http` or `https`, Home Assistant will fetch the `client_id` and check for `<link rel="redirect_uri" href="...">` HTML tags on the page. These URLs are not subjected to the same scheme validation and thus allow for arbitrary JavaScript execution on the Home Assistant administration page via usage of `javascript:` scheme URIs. This Cross-site Scripting (XSS) vulnerability can be executed on the Home Assistant frontend domain, which may be used