Steven Englehardt, Jeffrey Han and Arvind Narayanan:
We show that the simple act of viewing emails contains privacy pitfalls for the unwary. We assembled a corpus of commercial mailing-list emails, and find a network of hundreds of third parties that track email recipients via methods such as embedded pixels. About 30% of emails leak the recipient’s email address to one or more of these third parties when they are viewed. In the majority of cases, these leaks are intentional on the part of email senders, and further leaks occur if the recipi- ent clicks links in emails. Mail servers and clients may employ a variety of defenses, but we analyze 16 servers and clients and find that they are far from comprehen- sive. We propose, prototype, and evaluate a new defense, namely stripping tracking tags from emails based on en- hanced versions of existing web tracking protection lists.