1

In its default settings, uMatrix content blocker allows CSS and images (JPG, PNG, etc) from all sources, unless denied by a site-specific block list. It also allows 1st party cookies, scripts, and multimedia.

How much can a 3rd party learn about a site visitor through only CSS and images? Assume an up-to-date standards-compliant browser that lies about :visited status.

Obviously they receive HTTP headers (IP address, userAgent, etc) and can determine some browser features through media queries. They can tell if you choose to print, and possibly how long you keep the page loaded.

I think :hover pseudo-classes could (partially) track mouse movement.

What else can be done? Are the capabilities sufficient to provide unique fingerprinting across multiple sites, effectively a super-cookie?

  • Regardless of what type of resource you are requesting from a server, the browser can include any cookies for the server's domain. So if you already have cookies from evil.com and navigate to villains.com which has images from evil.com, evil.com can still track you with those cookies. – multithr3at3d Jul 21 at 21:57
  • But maybe you are already excluding 3rd party cookies. – multithr3at3d Jul 21 at 22:07
  • Yes, 3rd party cookies are blocked, and assume that none are previously set. – Foo Bar Jul 22 at 0:13
  • I don't really understand what the underlying "situation" is you are trying to explore. Exactly what control does the third party have? Normally the only time a 3rd party gets content on the page is for specific resources the 1st party requires - a JS library, a CSS utility, etc... If the 3rd party can put arbitrarily large amounts of content on the page, then they are capable of quite a lot even just with images/css, but such a scenario is almost never realistic in practice. – Conor Mancone Jul 22 at 10:12
  • 1
    @ConorMancone Content provider Alice.net is making use of Chuck.com (a popular service for content sites) by loading a few scripts and a CSS file. Chuck wants to track Alice's users, but they block 3rd party scripts & cookies. Can Chuck craft CSS that reveals data sufficient to track individuals across multiple Chuck-using sites? – Foo Bar Jul 22 at 14:32
1

This answer might be considered as a partial answer since it's not intended to be exhaustive.

With CSS only, a 3rd party can indeed (to a certain point) fingerprint a user's browser by:

  1. CSS querying by detecting some browser's unique properties (using prefixes. For example, Chrome & Safati present CSS properties prefixed with -webkit-)
  2. Retrieving system fonts / Font Inspection: For compatibility reasons, web browsers have fallback fonts for each font-family:

The collection script queries a device for installed fonts by generating hidden elements on a web page. The hidden element contains a predetermined character string and for each queried font, the height and width of the hidden element is compared to the height and width of an element containing the same string but formatted in the fallback font. If the height and width differs, it is be assumed that the font is installed.

  1. CSS Media Queries : Media queries can be used to gather almost the same amount of information as Window and Screen objects

Even if it's a bit out of scope, there is also "Canvas Fingerprinting":

That works by exploiting the HTML5 canvas element: when a user visits a website their browser is instructed to “draw” a hidden line of text or 3D graphic that is then rendered into a single digital token, a potentially unique identifier to track users without any actual identifier persistence on the machine.

Here is a pretty interesting piece of code that implement some CSS fingerprinting techs. And here (in chinese) a good paper on browser fingerprinting only with CSS.

0

As you are already aware, there are many ways to learn about users and their browsers using only CSS. As a result I'm just going to focus on the main aspect of your question: user tracking.

Fingerprinting is not necessary to make a super-cookie and track users because there are already plenty of non-cookie methods for tracking users. The two that come to mind are:

  1. Etags (see here and here)
  2. HSTS headers (see here)

Now, this would be a slam dunk if you had full control over the page, but I think that even as a third party either method of tracking (or ideally, both) would be quite doable.

Without full control over the page you'll have to rely on CSS tricks to get the browser to hit your tracking endpoints. As a quick description, HSTS tracking normally requires writing large numbers of image tags to the page, with each page from a slightly different sub-domain. This isn't possible if all the third party can do is control an included CSS file. However, there are tricks you can use to get CSS to load resources without having any control over the contents of the page (see here for one example), so I don't expect your position as a third-party should be a deal breaker for any of the methods above.

In short, ETAGS and HSTS headers both provide a way to create "super cookies" that allow you to track a user using only images or CSS. HSTS is a bit more theoretical, but ETAGS have actually been used for this purpose in the past.

I expect that, without much trouble, a 3rd party would be able to fully track users across the internet even with javascript and cookies fully disabled.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.