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COMPASS: Membership Inference Attack Against Vision Large Language Models via Adaptive Noise Calibration

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Summary

COMPASS is an adaptive gray-box membership inference attack against vision large language models (VLLMs) that calibrates both perturbation magnitude and membership-score direction using a known non-member reference set. The authors find that a low-noise regime often separates members from non-members better than large perturbations, with the score direction reversing in some model-dataset pairs. Tested on LLaVA v1.5-7B, MiniGPT-4 and HuluMed-32B, COMPASS reaches AUCs up to 0.88 on general-domain data and 0.64 on distribution-matched medical data, where the strongest baseline stays at chance.