No Caption, No Problem: Caption-Free Membership Inference via Model-Fitted Embeddings

• URL: http://arxiv.org/abs/2602.22689v1
• Date: Thu, 26 Feb 2026 07:07:11 GMT
• Title: No Caption, No Problem: Caption-Free Membership Inference via Model-Fitted Embeddings
• Authors: Joonsung Jeon, Woo Jae Kim, Suhyeon Ha, Sooel Son, Sung-Eui Yoon,
• Abstract summary: We propose MoFit, a caption-free MIA framework that constructs synthetic conditioning inputs explicitly overfitted to the target model's generative manifold.<n>MoFit consistently outperforms prior VLM-conditioned baselines and achieves performance competitive with caption-dependent methods.
• Score: 27.60095238548641
• License: http://creativecommons.org/licenses/by-nc-nd/4.0/
• Abstract: Latent diffusion models have achieved remarkable success in high-fidelity text-to-image generation, but their tendency to memorize training data raises critical privacy and intellectual property concerns. Membership inference attacks (MIAs) provide a principled way to audit such memorization by determining whether a given sample was included in training. However, existing approaches assume access to ground-truth captions. This assumption fails in realistic scenarios where only images are available and their textual annotations remain undisclosed, rendering prior methods ineffective when substituted with vision-language model (VLM) captions. In this work, we propose MoFit, a caption-free MIA framework that constructs synthetic conditioning inputs that are explicitly overfitted to the target model's generative manifold. Given a query image, MoFit proceeds in two stages: (i) model-fitted surrogate optimization, where a perturbation applied to the image is optimized to construct a surrogate in regions of the model's unconditional prior learned from member samples, and (ii) surrogate-driven embedding extraction, where a model-fitted embedding is derived from the surrogate and then used as a mismatched condition for the query image. This embedding amplifies conditional loss responses for member samples while leaving hold-outs relatively less affected, thereby enhancing separability in the absence of ground-truth captions. Our comprehensive experiments across multiple datasets and diffusion models demonstrate that MoFit consistently outperforms prior VLM-conditioned baselines and achieves performance competitive with caption-dependent methods.

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