So, I was asked to comment about clickjacking today. Technically, it isn’t a new vulnerability (IE6 fixed a variant in 2004, Firefox fixed a variant in September 2008), but a refinement of previous exploits and ennobled with a catchier name. It gained widespread coverage in October 2008 prior to the OWASP NYC conference when Jeremiah Grossman and Robert Hansen first said they would describe the vulnerability, then cancelled their talk for fear of unleashing Yet Another Exploit of Ultimate Doom.* The updated technique combines devious DOM manipulation with well-established attack patterns to make a respectable type of attack.
Clickjacking tricks a user into clicking on an attacker-supplied page while the user only sees the appearance and effect of clicking on a plain link. The attacker identifies an area in the target HTML that should receive the click event. This HTML is placed within an IFRAME such that the X and Y offsets of the frame place the target area in the upper left-hand corner of the frame’s visible area. This target IFRAME is visually hidden from the user (though the element remains part of the DOM). Then, the IFRAME is set within a second page (the content of which doesn’t matter) beneath the mouse cursor and, very importantly, dynamically moves to always be underneath the mouse. Then, when the user clicks somewhere within the second page the click is actually sent to the target area even though it appears to the user that the mouse is only above some innocuous link.Essentially, an attacker chooses some web page that, if the victim clicked some point (link, button, etc.) on that page, would produce some benefit to the attacker (e.g. generate click-fraud revenue, change a security setting, etc.). Next, the attacker takes the target page and places a second, innocuous page over it. The trick is to get the victim to make a mouse click on what appeared to be the innocuous page, but was actually an invisible element of the target page that has been automatically, but invisbly, placed beneath the cursor.The attack relies on luring a user to a server under the attacker’s control or a site that has been compromised by the attacker. Web site owners who ensure their site is free of cross-site scripting or other vulnerabilities can prevent their sites from being used as a relay point for the attacker. Yet other successful attacks, such as phishing, also rely on luring users to a server under the attacker’s control. The relative success of phishing implies that just securing web applications at the server isn’t the only solution because users can be tricked into visiting malicious web sites.The core of the attack occurs in the browser, which is where the real fix needs to appear. The problem is that browsers are intended to handle HTML from many sources and provide mechanisms to manipulate the location and visibility of elements within a web page. Consequently, any solution would have to block this attack while not inhibiting legitimate uses of this functionality.