Detecting Data Contamination in Large Language Models

• URL: http://arxiv.org/abs/2604.19561v1
• Date: Tue, 21 Apr 2026 15:13:30 GMT
• Title: Detecting Data Contamination in Large Language Models
• Abstract summary: Membership Inference Attacks (MIAs) aim to detect documents and whether they have been included in the training corpora of the Large Language Models (LLMs)<n>We study state-of-the-art (SOTA) MIAs under the black-box assumptions and compare them to each other using a unified set of datasets.<n>New method, called the Familiarity Ranking, was developed to showcase a possible approach to black-box MIAs.
• Score: 14.276221325597477
• License: http://creativecommons.org/licenses/by/4.0/
• Abstract: Large Language Models (LLMs) utilize large amounts of data for their training, some of which may come from copyrighted sources. Membership Inference Attacks (MIA) aim to detect those documents and whether they have been included in the training corpora of the LLMs. The black-box MIAs require a significant amount of data manipulation; therefore, their comparison is often challenging. We study state-of-the-art (SOTA) MIAs under the black-box assumptions and compare them to each other using a unified set of datasets to determine if any of them can reliably detect membership under SOTA LLMs. In addition, a new method, called the Familiarity Ranking, was developed to showcase a possible approach to black-box MIAs, thereby giving LLMs more freedom in their expression to understand their reasoning better. The results indicate that none of the methods are capable of reliably detecting membership in LLMs, as shown by an AUC-ROC of approximately 0.5 for all methods across several LLMs. The higher TPR and FPR for more advanced LLMs indicate higher reasoning and generalizing capabilities, showcasing the difficulty of detecting membership in LLMs using black-box MIAs.

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