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Vulnerability Overview / Description
The FortiGate SSL VPN Portal is prone to a reflected cross-site scripting (XSS) vulnerability. The HTTP GET parameter “redir” is vulnerable. An attacker can exploit this vulnerability by tricking a victim to visit a URL. The attacker is able to hijack the session of the attacked user, and use this vulnerability in the course of spear-phishing attacks, e.g. by displaying a login prompt that sends credentials of victim back to the attacker.
Note: This vulnerability is also an open redirect and is very similar to a vulnerability that was fixed in FortiOS in March 2016 (FG-IR-16-004).
Proof Of Concept
The following request exploits the issue: