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Clickjacking: Attacks and Defenses

David Lin-Shung Huang Lin-Shung Huang from Carnegie Mellon presents a study at USENIX Security about clickjacking attack vectors and the defenses to deploy for evading this issue.

Hello, I am David Lin-Shung Huang from Carnegie Mellon. Today I will be talking about clickjacking attacks and defenses and will introduce three new attack variants to show how all existing defenses are insufficient and present a new defense to address the root causes. So, this is joint work with Alex, Helen, Stuart from Microsoft Research, and Collin from Carnegie Mellon. This work was done while I was intern in a Microsoft Research.

So, to begin let me give an example of clickjacking on the web. Likejacking is type of clickjacking attacks that targets Facebook’s ‘Like’ button. So, suppose the user visits the attacker’s website. The attacker can embed Facebook’s ‘Like’ button on his page and the attacker wants to trick the user to click on the “Like” button, so, how can he do that? First, he can create a decoy button that lures the user to click on it to claim a free iPad.

Likejacking explained

Likejacking explained

Then, he can reposition the ‘Like’ button exactly on top of the decoy button and, finally, he can make the ‘Like’ button completely transparent using CSS, so, when the user tries to click on the decoy button he ends up getting tricked to click on something he didn’t intend to click on (see right-hand image).

The study outline in a nutshell

The study outline in a nutshell

So, in this talk first I’ll define clickjacking and characterize the existing attacks, then I’ll explain why existing defenses are insufficient and demonstrate three new attack variants that evade them. Finally, I’ll present a fundamental defense that addresses the root cause. We evaluated both attacks and defenses using Amazon Mechanical Turk. So, we’ll tell you how many people really fall for the fall for the attacks and how much the defenses can help.

So, you’ve seen an example of clickjacking but let’s define clickjacking in a more general way. Clickjacking may occur when multiple distrusting applications are sharing the same graphical display. So, an attack application compromises the context integrity of another application’s user interface when the user acts on the UI.

Basic essence of clickjacking

Basic essence of clickjacking

Let me explain what context integrity means with an example of a user clicking on the ‘Like’ button. First, the user is checking the target, and if he intends to click, he initiates the click. After a couple of hundred milliseconds the target is actually clicked. So, what is the context integrity in this case?

When the user is checking the target he recognizes that the target object, which is the ‘Like’ button, is what he intends to click, furthermore, he checks that the cursor feedback is where he intends to click, so, these two things are the visual integrity.

In addition, the target and pointer shouldn’t change from when he initiated the click to when the target is actually clicked. This is the temporal integrity. And we know that this type of problem is similar to the time of check to time of use problem except that now it’s the user performing the check. So, context integrity consists of both the visual integrity and the temporal integrity combined.

Hiding the target while compromising visual integrity

Hiding the target while compromising visual integrity

So, we surveyed a bunch of existing attacks, and now let me show you some examples of how they compromise context integrity. I’m using a PayPal checkout dialog, just for example (see left-hand image). One simple strategy to compromise the visual integrity is to hide the target. So, browsers allow web applications to make objects completely transparent using the CSS opacity. In some cases, the attacker may not need to or want to hide the entire target, the application can partially overlay important bits on the target to trick the user, and we provided more examples, such as propping, in our paper.

Manipulating the cursor feedback

Manipulating the cursor feedback

Another way to compromise the visual integrity is by manipulating the cursor feedback (see right-hand image). Browsers allow web applications to set custom cursor icons or even completely hide them, so, the attacker can set a cursor icon that displays the pointing hand away from the actual location of the pointer to mislead the user. So, when the user clicks on the decoy button the real pointer is actually clicking on the ‘Like’ button.

And as mentioned, context integrity consists of visual integrity and temporal integrity, so even if visual integrity isn’t compromised it’s still not enough to prevent all clickjacking attacks. The attacker can compromise temporal integrity using a technique we call Bait-and-Switch. The attacker baits the user to click on the decoy button which is the ‘Claim your FREE iPad’ button. When the user’s pointer hovers over the button the target is instantly repositioned under the pointer, and note that the display of the target object is fully visible and the pointer wasn’t altered but the user doesn’t have enough time to comprehend the visual change.
 

Read next: A Study of Clickjacking 2: Existing Defenses and New Attack Variants

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