Large Language Models (LLMs) have the promise to revolutionize computing broadly, but their complexity and extensive training data also expose significant privacy vulnerabilities. One of the simplest privacy risks associated with LLMs is their susceptibility to membership inference attacks (MIAs), wherein an adversary aims to determine whether a specific data point was part of the model's training set. Although this is a known risk, state of the art methodologies for MIAs rely on training multiple computationally costly shadow models, making risk evaluation prohibitive for large models. Here we adapt a recent line of work which uses quantile regression to mount membership inference attacks; we extend this work by proposing a low-cost MIA that leverages an ensemble of small quantile regression models to determine if a document belongs to the model's training set or not. We demonstrate the effectiveness of this approach on fine-tuned LLMs of varying families (OPT, Pythia, Llama) and across multiple datasets. Across all scenarios we obtain comparable or improved accuracy compared to state of the art shadow model approaches, with as little as 6% of their computation budget. We demonstrate increased effectiveness across multi-epoch trained target models, and architecture miss-specification robustness, that is, we can mount an effective attack against a model using a different tokenizer and architecture, without requiring knowledge on the target model.

本研究解决了大规模语言模型（LLMs）在成员推断攻击中的隐私风险，现有方法依赖多次训练计算开销大的影子模型，导致评估风险的成本高昂。本文提出了一种低成本的成员推断攻击方法，利用小型分位回归模型的集成，以有效判定文档是否属于训练集。实验结果显示，该方法在计算预算仅为现有方法6%的情况下，取得了与最先进方法相当或更好的精度，并在多种模型和数据集上展现了强大的有效性。 

大规模语言模型成员推断的数量级加速