Silverstripe External redirection risk in Security?ReturnURL

A vulnerability has been found in the SilverStripe framework where a login url can be potentially redirected to an external site.

For example, the url http://www.my-silverstripe-site.com/Security/login?BackURL=/\attacker-site.com will redirect successful logins to the page http://attacker-site.com. If that website were set up to look identical to the first with "login failed" then the user will likely just enter their user/pass again.