How to prevent open redirection attacks?

Question

Observation

 

                  Un-validated Redirect: The login form is vulnerable to un-validated redirect attacks:

 

Affected URLs: https://example.com/login.aspx?returnURL=https://www.attacker.com 

 

 

Impact 

 

    By modifying untrusted URL input to a malicious site, an attacker may successfully launch a phishing scam and steal user credentials.

 

 Recomendation

 

         Whitelist the redirect URLs and prevent redirection outside parent domain. 

 

 

 i dont know what is the problem is that.and what they saying.

My understading 

if login the application that redirect another un- validated page.

 

How to fix it what is the problem is that