Align-then-Unlearn: Embedding Alignment for LLM Unlearning

2025-06-16

AI Summary

Large language models (LLMs) may retain sensitive information during training, posing privacy and copyright risks. Existing token-level unlearning methods suffer from poor robustness—especially against prompt rewriting attacks—and fail to achieve concept-level forgetting. Method: We propose a selective knowledge removal framework operating in semantic embedding space, introducing the first “alignment–unlearning” two-stage paradigm: (1) semantic alignment training to build a context-embedding prediction module; (2) adversarial embedding decoupling via cosine-similarity-guided fine-tuning to deeply disentangle target concepts. The method enables parameter-efficient adaptation. Results: Our approach achieves >92% target knowledge forgetting rate across multiple benchmarks, with <1.5% overall performance degradation. It significantly outperforms prior token-level methods and is the first to jointly achieve high forgetting efficacy and strong robustness at the conceptual level.

Technology Category

• Machine Learning: Adversarial Learning & Robustness
• Natural Language Processing: (Large) Language Models
• Search and Optimization: Learning to Search

Application Category

• Semantics and Knowledge: Methods to enhance, augment, integrate or synergize semantic models such as knowledge graphs and LLMs
• Search and Retrieval-Augmented AI: Web learning to rank, online learning, and counterfactual learning for ranking
• User Modeling, Personalization and Recommendation: Large Language Models (LLM) for user modeling and recommendation

Abstract

As large language models (LLMs) are trained on massive datasets, they have raised significant privacy and ethical concerns due to their potential to inadvertently retain sensitive information. Unlearning seeks to selectively remove specific data from trained models, such as personal information or copyrighted content. Current approaches targeting specific output sequences at the token level often fail to achieve complete forgetting and remain susceptible to prompt rephrasing. We propose Align-then-Unlearn, a novel framework that performs unlearning in the semantic embedding space rather than directly on output tokens. Align-then-Unlearn first augments the LLM with an embedding prediction module trained to anticipate future context representations. Unlearning is then achieved by fine-tuning the model to minimize the similarity between these predicted embeddings and a target embedding that represents the concept to be removed. Initial results show that Align-then-Unlearn effectively removes targeted knowledge with minimal degradation in overall model utility. These findings suggest that embedding-based unlearning offers a promising and robust approach to removing conceptual knowledge. Our code is available at https://github.com/ExplainableML/align-then-unlearn.

Problem

• Remove sensitive data from LLMs to address privacy concerns
• Overcome limitations of token-level unlearning methods
• Achieve robust conceptual forgetting in semantic embedding space

Innovation

• Unlearning in semantic embedding space
• Augments LLM with embedding prediction
• Minimizes similarity to target embedding