Home > Articles > Home & Office Computing > The Web/Virtual Worlds/Social Networking

Wherever You Go, There They Are: The Perils of Cross-Site Tracking

In the early days of the Web, your web browser visited the server(s) of only a single company. Times have changed. Websites are intentionally instrumented to share information about your visit with third parties, including information aggregators, advertisers, media content suppliers, and free service providers. Greg Conti examines this practice and offers ways to block, or at least minimize, it.
Like this article? We recommend

If you think that when you visit a website you are only touching a single web server, think again. The most popular and less popular websites on the Web are intentionally instrumented to share information about your visit with third parties, including information aggregators, advertisers, media content suppliers, and free service providers.

Using a variety of techniques, the HTML and JavaScript contained within many web pages direct your browser to contact third parties, sometimes more than a dozen at a time. At best, you'll see only a flicker in the browser status bar as a sign that something is amiss.

Such tracking isn't new; chances are you first heard the term "web bug" in the late 90s, but since then cross-site tracking techniques have evolved into far more sophisticated, and far more prevalent, ways to track users as they hop from site to site across the Web. Because there is a relatively small number of tracking hubs but an ever increasing number of complicit web pages, third parties can observe your activities as you visit any of tens, hundreds, or even thousands of web sites.

Countermeasures exist, but unfortunately there is no single silver bullet. In fact, tracking techniques are integrated so tightly with the content available on the Web that separating the two is nearly impossible in many cases. If you want access to the content, tracking is part of the (Faustian) bargain.

Cross-site tracking thrives because webmasters insert small snippets of HTML and JavaScript into their web pages that contact third-party servers once the page is viewed by the end user. Including these snippets provides a payoff to the webmaster, such as providing free web analytics, affiliate network sales commissions, advertising revenue, social networking widgets, or content such as videos and maps. The strength of the Web is its intensely interconnected nature. Unfortunately, these interconnections facilitate and encourage cross-site tracking.

The information collected by these third parties is sensitive, including IP addresses, sites visited, individual pages viewed, and date and time of viewing. Because these services function by requesting the user's browser to contact their servers, the third party can tag the user's browser with a cookie.

More important, because large swaths of the web are instrumented to contact third parties, the largest third-party networks possess the capability to track users across a great deal of the Web. Thus it is possible to know much more about individuals' and organizations' web browsing history and behavior.

The information collected is extremely valuable and can be used for user profiling, data mining, and targeted advertising.

In addition, the mere existence of this tracking data makes it a ripe target for covetous attackers, governments, and competitors.

Prevalence on the Web

Instrumenting the Web to track the depth and breadth of individual web surfing is commonplace. Let's look at an example using the top 20 sites from Alexa's Kids and Teen website rankings (see Figure 1).

Figure 1

Figure 1 Third-party servers contacted when visiting popular children's websites.

There are numerous technical means to monitor third-party tracking. For this example, I chose one of the easiest. To gather this information I installed the Adblock Plus Firefox add-on and visited the websites one by one.

Adblock has a convenient feature that displays many forms of embedded content as each page is visited, and I used this technique to gather the data for Figure 1.

In the figure I listed my intended destination sites on the left and unintended destinations on the right. As you examine the figure, several important details stand out.

First consider the outdegree—the number of outbound connections from the intended destination. Outliers such as http://www.gamespot.com immediately emerge. Sites with a high number of outbound connections contact many third-party servers.

Some sites, such as Wikipedia, contact another domain (Wikimedia in this case), but the connections appear to be for legitimate processing requirements such as load balancing, not for suspect information sharing.

More important, consider the indegree—the number of inbound connections, for the third-party sites. The larger the indegree of a given third-party server, the greater that organization's visibility on the Web. In this example, google-analytics and doubleclick.net have the largest number of inbound connections.

It is important to note that you should consider these figures as a lower bound. There are numerous techniques, such as obfuscated JavaScript and DNS manipulations that can hide or mask third-party communications.

A detailed analysis at the packet level would yield additional insights and discoveries of previously undetected behind-the-scenes third-party communications.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020