Harry Potter is Still Here! Probing Knowledge Leakage in Targeted Unlearned Large Language Models

Bang Trinh Tran To, Thai Le


Abstract
This work presents LURK (Latent Unlearned Knowledge), a novel framework that probes for undesired knowledge retention in unlearned LLMs through adversarial suffix prompting. LURK automatically generates adversarial prompt suffixes designed to elicit residual knowledge about the Harry Potter domain, a commonly used benchmark for unlearning. Our experiments reveal that even models deemed successfully unlearned can leak idiosyncratic information under targeted adversarial conditions, highlighting critical limitations of current unlearning evaluation standards. By uncovering implicit knowledge through indirect probing, LURK offers a more rigorous and diagnostic tool for assessing the robustness of unlearning algorithms. Code and data will be available at https://github.com/Rachel1809/LURK.
Anthology ID:
2025.findings-emnlp.778
Volume:
Findings of the Association for Computational Linguistics: EMNLP 2025
Month:
November
Year:
2025
Address:
Suzhou, China
Editors:
Christos Christodoulopoulos, Tanmoy Chakraborty, Carolyn Rose, Violet Peng
Venue:
Findings
SIG:
Publisher:
Association for Computational Linguistics
Note:
Pages:
14427–14439
Language:
URL:
https://aclanthology.org/2025.findings-emnlp.778/
DOI:
Bibkey:
Cite (ACL):
Bang Trinh Tran To and Thai Le. 2025. Harry Potter is Still Here! Probing Knowledge Leakage in Targeted Unlearned Large Language Models. In Findings of the Association for Computational Linguistics: EMNLP 2025, pages 14427–14439, Suzhou, China. Association for Computational Linguistics.
Cite (Informal):
Harry Potter is Still Here! Probing Knowledge Leakage in Targeted Unlearned Large Language Models (To & Le, Findings 2025)
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https://aclanthology.org/2025.findings-emnlp.778.pdf
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