Lost in the Averages: A New Specific Setup to Evaluate Membership Inference Attacks Against Machine Learning Models

Membership Inference Attacks (MIAs) are widely used to evaluate the propensity of a machine learning (ML) model to memorize an individual record and the privacy risk releasing the model poses. MIAs are commonly evaluated similarly to ML models: the MIA is performed on a test set of models trained on datasets unseen during training, which are sampled from a larger pool, $D_{eval}$. The MIA is evaluated across all datasets in this test set, and is thus evaluated across the distribution of samples from $D_{eval}$. While this was a natural extension of ML evaluation to MIAs, recent work has shown that a record's risk heavily depends on its specific dataset. For example, outliers are particularly vulnerable, yet an outlier in one dataset may not be one in another. The sources of randomness currently used to evaluate MIAs may thus lead to inaccurate individual privacy risk estimates. We propose a new, specific evaluation setup for MIAs against ML models, using weight initialization as the sole source of randomness. This allows us to accurately evaluate the risk associated with the release of a model trained on a specific dataset. Using SOTA MIAs, we empirically show that the risk estimates given by the current setup lead to many records being misclassified as low risk. We derive theoretical results which, combined with empirical evidence, suggest that the risk calculated in the current setup is an average of the risks specific to each sampled dataset, validating our use of weight initialization as the only source of randomness. Finally, we consider an MIA with a stronger adversary leveraging information about the target dataset to infer membership. Taken together, our results show that current MIA evaluation is averaging the risk across datasets leading to inaccurate risk estimates, and the risk posed by attacks leveraging information about the target dataset to be potentially underestimated.