Overfitting, robustness, and malicious algorithms: A study of potential causes of privacy risk in machine learning.

Machine learning algorithms, when applied to sensitive data, pose a distinct threat to privacy. A growing body of prior work demonstrates that models produced by these algorithms may leak specific private information in the training data to an attacker, either through the models' structure or their observable behavior. This article examines the factors that can allow a training set membership inference attacker or an attribute inference attacker to learn such information. Using both formal and empirical analyses, we illustrate a clear relationship between these factors and the privacy risk that arises in several popular machine learning algorithms. We find that overfitting is sufficient to allow an attacker to perform membership inference and, when the target attribute meets certain conditions about its influence, attribute inference attacks. We also explore the connection between membership inference and attribute inference, showing that there are deep connections between the two that lead to effective new attacks. We show that overfitting is not necessary for these attacks, demonstrating that other factors, such as robustness to norm-bounded input perturbations and malicious training algorithms, can also significantly increase the privacy risk. Notably, as robustness is intended to be a defense against attacks on the integrity of model predictions, these results suggest it may be difficult in some cases to simultaneously defend against privacy and integrity attacks.