When the cookie meets the blockchain:
Privacy risks of web payments via cryptocurrencies
(Princeton researchers Dillon Rewsman, Steven Goldfeder, Harry Kalender and Arvind Narayanan)
We show how third-party web trackers can deanonymize users of cryptocurrencies. We present two distinct but complementary attacks. On most shopping websites, third party trackers receive information about user purchases for purposes of advertising and analytics. We show that, if the user pays using a cryptocurrency, trackers typically possess enough information about the purchase to uniquely identify the transaction on the blockchain, link it to the user’s cookie, and
further to the user’s real identity. Our second attack shows that if the tracker is able to link two purchases of the same user to the blockchain in this manner, it can identify the user’s entire cluster of addresses and transactions on the blockchain, even if the user employs blockchain anonymity techniques such as CoinJoin. The attacks are passive and hence can be retroactively applied to past purchases. We discuss several mitigations, but none are perfect.
Of 130 online merchants that accept Bitcoin, the researchers say, 53 leak payment information to 40 third parties, “most frequently from shopping cart pages,” and most of these on purpose (for advertising, analytics and the like).
Of the 130 sites the researchers checked:
In total, 107 sites leaked some kind of transaction information;
31 allowed third-party scripts to access users’ Bitcoin addresses;
104 shared the non-BTC denominated price of a transaction; and
30 shared the transaction price in Bitcoin.
A total of 49 merchants shared users’ identifying information, and 38 shared that even if the user tries to stop them with tracking protection.
Blog of one of the researchers talking about it: