November 23, 2017

I never signed up for this! Privacy implications of email tracking

In this post I discuss a new paper that will appear at PETS 2018, authored by myself, Jeffrey Han, and Arvind Narayanan.

What happens when you open an email and allow it to display embedded images and pixels? You may expect the sender to learn that you’ve read the email, and which device you used to read it. But in a new paper we find that privacy risks of email tracking extend far beyond senders knowing when emails are viewed. Opening an email can trigger requests to tens of third parties, and many of these requests contain your email address. This allows those third parties to track you across the web and connect your online activities to your email address, rather than just to a pseudonymous cookie.

Illustrative example. Consider an email from the deals website LivingSocial (see details of the example email). When the email is opened, client will make requests to 24 third parties across 29 third-party domains.[1] A total of 10 third parties receive an MD5 hash of the user’s email address, including major data brokers Datalogix and Acxiom. Nearly all of the third parties (22 of the 24) set or receive cookies with their requests. In a webmail client the cookies are the same browser cookies used to track users on the web, and indeed many major web trackers (including domains belonging to Google, comScore, Adobe, and AOL) are loaded when the email is opened. While this example email has a large number of trackers relative to the average email in our corpus, the majority of emails (70%) embed at least one tracker.

How it works. Email tracking is possible because modern graphical email clients allow rendering a subset of HTML. JavaScript is invariably stripped, but embedded images and stylesheets are allowed. These are downloaded and rendered by the email client when the user views the email.[2] Crucially, many email clients, and almost all web browsers, in the case of webmail, send third-party cookies with these requests. The email address is leaked by being encoded as a parameter into these third-party URLs.

Diagram showing the process of tracking with email address

When the user opens the email, a tracking pixel from “tracker.com” is loaded. The user’s email address is included as a parameter within the pixel’s URL. The email client here is a web browser, so it automatically sends the tracking cookies for “tracker.com” along with the request. This allows the tracker to create a link between the user’s cookie and her email address. Later, when the user browses a news website, the browser sends the same cookie, and thus the new activity can be connected back to the email address. Email addresses are generally unique and persistent identifiers. So email-based tracking can be used for targeting online ads based on offline activity (say, to shoppers who used a loyalty card linked to an email address) and for linking different devices belonging to the same user.

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Why King George III Can Encrypt

[This is a guest post by Wenley Tong, Sebastian Gold, Samuel Gichohi, Mihai Roman, and Jonathan Frankle, undergraduates in the Privacy Technologies seminar that I offered for the second time in Spring 2014. They did an excellent class project on the usability of email encryption.]

PGP and similar email encryption standards have existed since the early 1990s, yet even in the age of NSA surveillance and ubiquitous data-privacy concerns, we continue to send email in plain text.  Researchers have attributed this apparent gaping hole in our security infrastructure to a deceivingly simple source: usability.  Email encryption, although cryptographically straightforward, appears too complicated for laypeople to understand.  In our project, we aimed to understand why this problem has eluded researchers for well over a decade and expand the design space of possible solutions to this and similar challenges at the intersection of security and usability.

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Groklaw Shuts Down, Citing NSA Eavesdropping

The legendary technology law blog Groklaw is shutting down. Groklaw’s founder and operator, Pamela “PJ” Jones, wrote that in light of current eavesdropping, email is no longer secure. She went on to say:

There is no way to do Groklaw without email. Therein lies the conundrum.
[…]
What to do? I’ve spent the last couple of weeks trying to figure it out. And the conclusion I’ve reached is that there is no way to continue doing Groklaw, not long term, which is incredibly sad. But it’s good to be realistic. And the simple truth is, no matter how good the motives might be for collecting and screening everything we say to one another, and no matter how “clean” we all are ourselves from the standpoint of the screeners, I don’t know how to function in such an atmosphere. I don’t know how to do Groklaw like this.

I can’t help thinking that there might be more here than meets the eye.
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