Princeton University researchers analyzed 130 Bitcoin-friendly e-commerce sites from 21 countries that allow users to pay with Bitcoin. Researchers looked at how these sites handle Bitcoin transactions and what information leaks during the checkout process. They found that 53/130 of merchants leaked payment information to third parties, most frequently from shopping cart pages. The research paper titled “When the cookie meets the blockchain: Privacy risks of web payments via cryptocurrencies”” is available in the research paper. It is not the first paper that deals with Bitcoin’s privacy issues; previous work can be found here.”
Source: https://www.bleepingcomputer.com/news/technology/ad-trackers-on-e-commerce-sites-can-unmask-bitcoin-transactions/