Over the past decade, our financial transaction data has become one of the most sought-after data sets. Credit card processors like American Express, Mastercard, and Visa are at the center of it.
All of this is happening under a veil of secrecy. Credit card companies may acknowledge that they make money from analyzing transactions, but they are vague about what data they actually share […] Even Apple, which prohibits Goldman Sachs from using its card data for marketing purposes, couldn’t get the same concessions out of Mastercard, its card network.
Here’s a link to the study mentioned in the article, where MIT researchers successfully de-anonymized financial data that these companies claim had privacy protections.