When the cookie meets the blockchain: Privacy risks of web payments via cryptocurrencies

TL;DR

This paper demonstrates how third-party web trackers can deanonymize cryptocurrency users by linking online purchase data to blockchain transactions, revealing sensitive user information and transaction clusters.

Contribution

It introduces two novel passive attacks that connect web tracking data with blockchain transactions, exposing privacy vulnerabilities in cryptocurrency payments.

Findings

Trackers can link purchases to blockchain transactions and user identities.

Attackers can identify entire address clusters even with anonymity techniques.

Passive attacks can be applied retroactively to past transactions.

Abstract

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.