Strong Membership Inference Attacks on Massive Datasets and (Moderately)
  Large Language Models
Authors: name: Jamie Hayes; name: Ilia Shumailov; name: Christopher A. Choquette-Choo; name: Matthew Jagielski; name: George Kaissis; name: Katherine Lee; name: Milad Nasr; name: Sahra Ghalebikesabi; name: Niloofar Mireshghallah; name: Meenatchi Sundaram Mutu Selva Annamalai; name: Igor Shilov; name: Matthieu Meeus; name: Yves-Alexandre de Montjoye; name: Franziska Boenisch; name: Adam Dziedzic; name: A. Feder Cooper
Published: 2025-05-24 16:23:43

Abstract: State-of-the-art membership inference attacks (MIAs) typically require
training many reference models, making it difficult to scale these attacks to
large pre-trained language models (LLMs). As a result, prior research has
either relied on weaker attacks that avoid training reference models (e.g.,
fine-tuning attacks), or on stronger attacks applied to small-scale models and
datasets. However, weaker attacks have been shown to be brittle - achieving
close-to-arbitrary success - and insights from strong attacks in simplified
settings do not translate to today's LLMs. These challenges have prompted an
important question: are the limitations observed in prior work due to attack
design choices, or are MIAs fundamentally ineffective on LLMs? We address this
question by scaling LiRA - one of the strongest MIAs - to GPT-2 architectures
ranging from 10M to 1B parameters, training reference models on over 20B tokens
from the C4 dataset. Our results advance the understanding of MIAs on LLMs in
three key ways: (1) strong MIAs can succeed on pre-trained LLMs; (2) their
effectiveness, however, remains limited (e.g., AUC<0.7) in practical settings;
and, (3) the relationship between MIA success and related privacy metrics is
not as straightforward as prior work has suggested.