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Towards Source-Free Machine Unlearning
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Proposes a source-free machine unlearning method that estimates the retain-data Hessian from only the forget data and trained model, enabling zero-shot unlearning for linear and mixed-linear classifiers with bounded-error guarantees.
Problem
Machine unlearning when the original training (remaining) data is unavailable, so the retain Hessian needed for Newton-style unlearning cannot be computed directly.
Approach
Approximates the retain Hessian by solving a semidefinite program that matches second-order loss differences at random perturbed points, substituting forget-set loss differences for retain-set ones under a bounded-error assumption, and proves a Frobenius-norm error bound and a gradient-norm bound on the unlearned model.
Key results
- On CIFAR-10 with a linear classifier and 10% forget data, Unlearned(-) reaches 70% test / 71% remaining / 68% forget vs. 72/74/72 for Retrained, outperforming NegGrad, Random Labels, JiT, and Adversarial baselines (around 51-52% test).
- With 5% forget data on CIFAR-10, the performance gap to retraining is 0%, while at 20% on mixed-linear CIFAR-100 the gap reaches up to 6.6% on remaining data.
- Increasing the number of perturbations (250 to 1000) and the L2 regularization lambda (0 to 0.001) progressively closes the gap to the retrained model.
Measured effects reported in the paper
| What was changed | Where | Measured | Effect |
|---|---|---|---|
| use source-free Hessian estimation for unlearning | linear classifier on ResNet-18 features · CIFAR-10 | test accuracy: 72% -> 70% | Hurt |
| increase forget data proportion from 5% to 15% (ablation) | Unlearned (-) · CIFAR-10 | test accuracy: 73% -> 59% | Hurt |
| increase forget data proportion from 5% to 15% (ablation) | Unlearned (-) · CIFAR-10 | MIA score: 49.4% -> 55.8% | Mixed |
| increase Hessian-estimation perturbations from 250 to 1000 (ablation) | Unlearned (-) · CIFAR-10 | test accuracy: 57% -> 72% | Helped |
| increase Hessian-estimation perturbations from 250 to 1000 (ablation) | Unlearned (-) · CIFAR-10 | remaining data accuracy: 58% -> 74% | Helped |
| increase L2 regularization λ from 0 to 0.001 (ablation) | Unlearned (-) · CIFAR-10 | test accuracy: 70% -> 72% | Helped |
Limitations
- Theory applies to linear classifiers with convex losses; deep networks are handled only via linearization (mixed linear).
- Performance gap to retraining widens as forget-set size grows (e.g., 14% at 10% in the first CIFAR-10 table row set).
- Storing and inverting the full Hessian is computationally impractical for large models.
Experimental setup
Datasets: CIFAR-10 CIFAR-100 Stanford Dogs Caltech-256
Benchmarks: CIFAR-10 CIFAR-100 Stanford Dogs Caltech-256
Metrics: test accuracy remaining accuracy forget accuracy membership inference attack score
Baselines: Retrained Unlearned(+) NegGrad Random Labels JiT Adversarial
Builds on: Guo et al. certified data removal (Newton update) Mixed Linear Unlearning zero-shot machine unlearning Just-in-Time unlearning
Closest papers by meaning
- The Utility and Complexity of In- and Out-of-Distribution Machine Unlearning (ICLR 2025 · similarity 0.92)
- Hessian-Free Online Certified Unlearning (ICLR 2025 · similarity 0.91)
- Efficient Source-free Unlearning via Energy-Guided Data Synthesis and Discrimination-Aware Multitask Optimization (ICML 2025 · similarity 0.91)
- Certified Unlearning for Neural Networks (ICML 2025 · similarity 0.91)
- A Certified Unlearning Approach without Access to Source Data (ICML 2025 · similarity 0.91)
- Rewind-to-Delete: Certified Machine Unlearning for Nonconvex Functions (NeurIPS 2025 · similarity 0.91)
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