The concern with Open Redirect flaws is that your application could be redirecting the user to an external and possibly malicious site. Whenever you do a redirect, you should always check that the URL is local. There are a couple of ways in .NET you can do this leveraging UrLHelper.IsLocalUrl() (https://msdn.microsoft.com/en-us/library/gg402075(v=vs.118).aspx) or RequestExtensions.IsUrlLocalToHost() (https://docs.microsoft.com/en-us/previous-versions/aspnet/web-frameworks/gg569095(v=vs.111)) . There are some example usages you can reference here: https://docs.microsoft.com/en-us/aspnet/mvc/overview/security/preventing-open-redirection-attacks .

If you're redirecting the user to another domain, I would recommend you create a white-list of domains that the application could redirect to. Simply check the URL against the white-list before doing the redirection. This will ensure that the user will only be redirected to trusted domains that you have defined. For external domains, it is best practice to let the user know before the redirection occur that they are being redirected so that they are aware they are leaving your application.