The plugin does not validate, sanitise and escape the redirect_to parameter before using it to redirect user, either via a Location header, or meta url attribute, when the Rename Login Page is active, which could lead to an Arbitrary Redirect as well as Cross-Site Scripting issue. Exploitation of this issue requires the Login Page URL value to be known, which should be hard to guess, reducing the risk
With a logged in user:
https://example.com/xxxxxx?redirect_to=https://wpscan.com
https://example.com/xxxxxx?redirect_to="><script>alert(/XSS/)</script>