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content-blocker plugin enabled and extract some possible implications of this finding.
To be able to counteract those implications, companies
started using some anti-adblocker systems, mechanisms that
detect when a user is using an adblocker and correspondingly
block the service for that specific user. In [12], Iqbal et al. explore the prevalence of such systems and the performance of
anti-adblocker filter lists created by the community to avoid
being blocked while using an adblocker.
Another way of improving sales profit is not directly with
targeted advertisements but with price discrimination. The
underlying idea is that, depending on the user profile collected
by the tracking mechanisms (living location, wealth, relatives,
friends and other related characteristics), the price that people
are willing to pay for the same object is different. In [13], Mikians et al. perform a study on how to detect the price and search
discrimination on the Internet using a collaborative approach
by means of a browser plugin.

Privacy/Security
Krishnamurthy et al. was one of the first works that tried to
measure the impact of web tracking in privacy and explore
some measures to regain it [14]. The research also includes
a study about the quality and usability modifications introduced by the tools used to improve privacy.
On the other hand, Ikram et al. focus their research on
exploring the concept of implicit chain of trust within thirdparties [15]. The idea is that when a website imports a resource
from a third-party, the imported resource can also embed resources from other different third-parties that the original
website does not necessarily know about. They found that although most of the websites' chain of trust is relatively small,
a high percentage of them end up loading resources from a
suspicious website or, in other words, a website that has been
categorized as a possible threat to privacy or security.

Protection Measures
Currently, the only effective way of protecting the user against
web tracking is by means of content-blockers, a type of plugin
installable in the browser that examines all of the URLs being accessed by the browser. Those URLs are then compared
to a black list or a database containing the known web tracking websites, and if the comparison is positive, the resource is
blocked. There are other methods like Javascript or Flash blockers to improve the security even more, but they usually break
the website layout, degrading the usability or even making
the website inaccessible which is a price too high to pay for
the common user. Thus, content-blockers have become the
de-facto solution for the privacy problem. Examples of popular content blockers include AdBlock Plus, Ghostery and uBlock
Origin.

Challenges/Open Issues
Using content-blockers as the main countermeasure is a defensive approach which limits the security to the already known
websites. In an environment where new web tracking methods
December 2020	

constantly emerge, and creating a new website or domain is
done in a matter of minutes, it is not the ideal approach. Even
so, there are some important challenges and open issues to be
able to block web tracking in a more proactive way.
The first and foremost challenge we must deal with is the
constant evolution of web tracking methods, using more and
more complex and intrusive techniques specifically designed
to overcome the privacy protection mechanisms available
within the browser. Usually, there is very little, or no information at all, about those new web tracking mechanisms. To find
them requires experts on the topic who are able to inspect the
code and the new functions included in successive updates of
the website programming frameworks, looking for parameters and characteristics that could be used to track the users.
Once found, to check for empirical verification, an experiment
should be executed. This process is very hard and time-consuming, which impacts on the ability of researchers to detect
new methods.
Another obstacle is how to deal with minification and obfuscation. The first is a technique that tries to minimize the
resource loading times eliminating white spaces, break lines
and shortening the variable names of the resource code. The
second one is similar but additionally changes the code structure and renames the variables by non-sense strings to make
the code almost unreadable by humans. These two techniques
render traditional methods based on code inspection unfeasible in a large population of websites, making it even harder to
solve the first challenge of finding new web tracking methods.
To improve the state-of-the-art on these two aspects, first
we need to take network measurements about the resources
loaded and shared by first and third-party trackers, to be able
to detect patterns and common characteristics between the
web tracking code. In addition, we need to take those measurements on a large scale to be able to use other techniques, like
machine-learning or data-mining algorithms, to automatically
explore the information without requiring human intervention. This presents the third challenge, the lack of public
datasets with detailed information and measurements about
the resources loaded and parsed by websites at a large scale,
that would permit researchers to detect new web tracking
methods in a more automated way than it is currently done.

ORM: A Large-scale Web Measurement
Framework
In this paper, we introduce Online Resource Mapper (ORM),
an open source large-scale web measurement framework specifically designed to address the challenges described in the
previous section. ORM is an active measurement framework
and as such has been developed to be able to crawl information
directly from websites about every resource accessed, either
loaded by the main website or embedded within a third-party
tracker resource. Moreover, the system automatically tries to
unminify/unobfuscate the code of the resources found to ease
their study, if necessary.
ORM uses a combination of Selenium, Chromium and the
Chrome DevTools protocol to get detailed information about all

IEEE Instrumentation & Measurement Magazine	53



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