arXiv Machine Learning By Shengjie Niu, Yeheng Ge, Jian Huang

Black-Box Membership Inference via Word-Level Probability Estimation

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The paper introduces Word-level Probability MIA (WPMIA), a black-box membership inference attack that estimates word-level generation probabilities via Monte Carlo sampling and local kernel smoothing, then aggregates them into a sequence-level likelihood estimator. By conditioning on different prefixes, WPMIA amplifies distributional differences between member and non-member texts, outperforming existing black-box baselines on open-source LLMs and achieving an average TPR@5%FPR of 42.0 on proprietary models such as GPT‑5‑Chat, Gemini‑2.5‑Flash, and Claude‑4.5‑Haiku.

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arXiv Machine Learning
3d ago

From Training Exposure to Membership Evidence: A Controlled Study of OLMo 2

The study re‑runs the OLMo 2 1B language model with controlled training exposure, swapping selected training chunks with matched documents to assess membership inference. Using a counterfactual run from the same seed, the authors find that a single exposure modestly increases a document’s likelihood but rarely allows reliable identification; the reference‑ratio method achieves an AUC of 0.767 and detects 3.8 % of members at a 1 % false‑positive rate. Even with matched references, document‑level variability limits individual inference, and the authors plan to release checkpoints, log‑probabilities, and evaluation data.

By Ali Banaei, Muhammad Shahab Shahzad, Gerald Q. Maguire Jr., Marco Chiesa, Dejan Kosti\'c