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A Study of Clickjacking 3: Ensuring Visual and Temporal Integrity

This part of Lin-Shung Huang’s presentation covers new techniques that are effective for ensuring better defenses against different variants of clickjacking.

Key goals for better defenses

Key goals for better defenses

Now, we know that current defenses are insufficient in one way or another. The question is: can we design a better defense? We set a few design goals: the defense should support embedding third-party objects such as Facebook’s ‘Like’ button; the defense should not prompt users for each of their actions and force users to make bad security decisions; it should also not break existing sites and should be resilient to new attack vectors.

InContext defense explained

InContext defense explained

So, InContext is a set of techniques to ensure context integrity for user actions (see left-hand image). Since we don’t want to break existing sites, we use a server opt-in approach. We let the websites indicate their sensitive UIs, for example, Facebook can mark their ‘Like’ buttons as sensitive UI. Then we let browsers enforce context integrity when users act on those sensitive UIs.

Ensuring target's visual integrity

Ensuring target’s visual integrity

So, how can we ensure visual integrity of the target UIs? The basic idea is to ensure that what the user sees is what the sensitive UI intends to display (see right-hand image). We render these sensitive objects off screen in isolation of other websites and applications and we make sure that the screen image displayed to the user matches the reference rendering pixel by pixel.

We implemented a prototype of this on top of IE 9 and measured that the processing delay during each click is less than 30 milliseconds in the worst test case. So, without any code optimization the performance cause is pretty reasonable and only occurs when the user clicks.

Ensuring pointer’s visual integrity

Ensuring pointer’s visual integrity

Now, how can we ensure visual integrity of the pointer? This is trickier (see right-hand image). One strategy is just to remove cursor customization. So even if the attacker renders a fake cursor – the real cursor is always displayed and as you can see, again, there are two cursors moving on the screen.

We found that this reduced the attack’s success rate from 43% down to 16% but still some users may be confused. Another disadvantage of this approach is that legitimate sites won’t be able to hide the cursors, for example, when playing a movie in a full screen showing a cursor would be annoying.

A better strategy

A better strategy

So, are there better defenses that can still allow cursor customization and provide visual integrity for the pointer? One method is to freeze the screen outside of the target display area when the real pointer enters the target (see left-hand image). So, for example, when the pointer enters the red area which is the sensitive UI, the screen freezes, including the video ad and the fake cursor, so you see the fake cursor is hanging outside of the ‘Skip this ad’ link.

The attack’s success rate reduced down to 15% and it’s similar to removing cursor customization. We tested this defense again but enlarging the target area with the margin of 10px. And this defense reduced the attack’s success rate down to 12%. We tested this defense again, now with the margin of 20px. This defense reduced the attack’s success rate down to 4%, which is lower than the baseline. The baseline is that around 5% of the users, we found that they would click on the webcam access button even under no attacks.

So, this suggests that having a sufficient margin around the sensitive UI is very important in addition to screen freezing and it allows the users to notice when their pointer enters the sensitive UI.

Utilizing the lightbox effect

Utilizing the lightbox effect

We also tested a more obtrusive defense by rendering lightbox effects when the pointer enters the target (see right-hand image). Notice that the pixels of the grey mass are randomized, otherwise an attacker may be able to adjust its intensity to cancel the lightbox effect.

And to our surprise, we didn’t observe significant improvement using the lightbox effect, perhaps it’s because the screen freezing itself was already very effective. So, since screen freezing is less obtrusive, we leave the lightbox effect as an optional defense.

An outline of methods to enforce temporal integrity

An outline of methods to enforce temporal integrity

Now, how do we enforce temporal integrity? (see left-hand image) The normal flow of user click is like this: the user checks the target, the user clicks the target and then the system delivers the target, the click to the target application.

We enforce temporal integrity before the system delivers the click to the target application. We can use two techniques. One is the UI delay, which is, after visual changes on target or pointer – we invalidate clicks for a number of milliseconds. Another technique is pointer re-entry. So, after visual changes on target we invalidate clicks until the pointer re-enters the target.

UI delay as a technique

UI delay as a technique

So, first let’s see what UI delay does (see right-hand image). When the target UI or pointer changes we invalidate clicks for X amount of milliseconds. In the case of double-click attacks, the second click would be invalidated because the target or dialog just appeared. We found that using UI delays up to 250 milliseconds and 500 milliseconds were both very effective against this specific attack. Only less than 2% of the users would still fall for the attack. This suggests that most of the users are double-clicking faster than 500 milliseconds therefore they are protected by this defense.

Pointer re-entry

Pointer re-entry

In some cases setting the right amount of UI delay is difficult, and so we have an alternative solution to enforce temporal integrity. Whenever the UI changes, we invalidate clicks until the pointer re-enters the target (see left-hand image). So, as you can see in this example, after the double-click clicks are not delivered if the pointer did not explicitly enter the target. The dialog is enabled only after the pointer re-enters the target.

We found that the pointer re-entry was extremely effective, it prevented all of our users from falling for the double-click attack. This approach does not require applications to set a certain amount of UI delay, and advantage of pointer re-entry is that it cannot be gamed by attacker who can anticipate the time of user clicks, for example, the attacker might try to display the target 500 milliseconds before the user clicks, that would probably defeat a 500 millisecond UI delay. However, this technique doesn’t apply to touch screens because touch screens don’t have a cursor feedback.

Whack-a-mole attack in a nutshell

Whack-a-mole attack in a nutshell

So, we’ve shown that our defenses are effective against cursor spoofing and double-click attacks individually. We wanted to see whether our defenses would still hold against a combined attack. So, as I mentioned the Whack-a-mole attack (see right-hand image) combines cursor spoofing and fast-paced object clicking techniques.

We excluded victims who were moving their pointers in and out the ‘Like’ button for many seconds and deliberating whether or not to click. This is because clickjacking defenses have no chance of stopping scenarios where users intentionality decided to click on the target.

So, we found that our defenses combined were able to reduce the attack down to 1%. The pointer entry delay presented here is a UI delay that invalidates clicks for X amount of seconds after the pointer enters the target. We observed that this pointer entry delay is crucial to reducing the attack success rate, and a larger amount of delay is definitely better.

To our surprise, we observed that the social engineering aspects of this attack are still very effective although they are out of scope.
 

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

Read next: A Study of Clickjacking 4: Summary and Q&A

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