Any web application that redirects to a URL that is specified via the request such as the querystring or form data can potentially be tampered with to redirect users to an external, malicious URL. This tampering is called an open redirection attack.

Whenever your application logic redirects to a specified URL, you must verify that the redirection URL hasn't been tampered with.

To understand the vulnerability, let's look at how the login redirection works in a default ASP.NET MVC 2 Web Application project. In this application, attempting to visit a controller action that has the [Authorize] attribute will redirect unauthorized users to the /Account/LogOn view. This redirect to /Account/LogOn will include a returnUrl querystring parameter so that the user can be returned to the originally requested URL after they have successfully logged in.

Microsoft: Preventing Open Redirection Attacks