An open redirect is a vulnerability that exists when a script allows redirectionto an external site by directly calling a specific URL in an unfiltered,unmanaged fashion, which could be used to redirect victims to unintended,malicious web sites. A web application accepts a user-controlled input that specifies a link to an external site, and uses that link in a Redirect.
A similar vulnerability is reported in Google by "Ucha Gobejishvili ( longrifle0x )". This problem may assist an attacker to conduct phishing attacks, trojan distribution, spammers.
Same vulnerability in Facebook, Discovered by ZeRtOx from Devitel group:
Impact of Vulnerability :
The user may be redirected to an untrusted page that contains malware which may then compromise the user's machine. This will expose the user to extensive risk and the user's interaction with the web server may also be compromised if the malware conducts keylogging or other attacks that steal credentials, personally identifiable information (PII), or other important data.
The user may be subjected to phishing attacks by being redirected to an untrusted page. The phishing attack may point to an attacker controlled web page that appears to be a trusted web site. The phishers may then steal the user's credentials and then use these credentials to access the legitimate web site.