REVIEW 3 major objections 5 minor 35 references
Recalling The Forgotten Class Memberships: Unlearned Models Can Be Noisy Labelers to Leak Privacy
T0 review · 3 major / 5 minor · reviewed 2026-08-06 · deepseek-v4-flash
Pith's one-line read The paper claims that unlearned models retain enough class-level knowledge for an attacker to recover the class memberships of supposedly forgotten data, using only the unlearned model and no original trained model.
desk verdict The ULM-only recall attack is a genuinely new scenario and the experiments are thorough, but the missing retrained-model baseline leaves the privacy-leak claim unproven. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The machinery is a two-stage alternating loop. In the denoising knowledge-distillation stage, the unlearned model (teacher) labels mixup-augmented images from the prediction set, the student model is trained on the mixed soft labels, and the student gradually stops trusting the teacher's noise. In the confident membership recall stage, both models score every prediction-set image, their smoothed probability vectors are multiplied elementwise, and the top-K highest joint-probability samples per class are treated as confidently recalled memberships and used to retrain the student (and, in the open-source case, also the teacher). The joint-probability agreement is the load-bearing filter: it assumes that a forgotten instance will still draw high probability from both models for its true class, while noise will not be consistently agreed upon.
What would settle it
A decisive check is to run MRA against SalUn or UNSC with a prediction set containing only forgotten images and no test-set anchors; if forgetting-set accuracy stays near the unlearned baseline instead of jumping, the reported recall is an artifact of anchor leakage. A second decisive check is to unlearn by removing the target classes from the output space entirely; if MRA then cannot assign the old class labels, the attack's success depends on the class remaining in the label space.
Extended reading notes
Core claim
The central discovery is that unlearned models retain enough class-level knowledge to act as noisy labelers for their own forgotten data. The paper demonstrates that a teacher–student distillation loop, combined with a balanced top-K selection of high-confidence teacher–student agreements, turns an unlearned model into a recalled model that assigns correct labels to forgotten instances. The effect is strongest for precisely unlearning methods: in the open-source case, MRA drives forgetting-set accuracy from 0.117 to 0.997 for SalUn on CIFAR-10 and from 0.209 to 0.985 for UNSC on CIFAR-100. Even in the closed-source case, where only black-box predictions are available, the recalled model beats the unlearned model on every dataset and every unlearning method tested.
Load-bearing premise
The attack depends on the forgotten classes still being among the model's possible answers and on the attacker holding some non-forgotten test images from those same classes to anchor confident label selection; without those anchors, the unlearned model's own guesses on forgotten data are too noisy for the agreement filter to start.
Editorial extensions
If this is right
- Black-box unlearning APIs are vulnerable: querying the unlearned model on a mixed prediction set and running the agreement loop can recover class membership of supposedly erased images.
- Precise unlearning methods that preserve non-forgotten performance are the most exposed, because their pseudo-labels carry the least noise.
- Unlearning evaluation should treat near-zero accuracy on the forgetting set as insufficient evidence of privacy; class-recall attacks like MRA measure a different residual.
- Releasing unlearned model weights multiplies the risk, since updating the teacher during MRA produces the largest recovery gains.
- MRA gives a model-agnostic benchmark for comparing MU methods on privacy leakage, applicable across low- and high-resolution image datasets.
Reading between the lines
- Inference: MRA can be read as a measurement of residual class-level knowledge, so it could be repurposed as an auditing tool for unlearning, a use the paper does not develop.
- Inference: A practical defense suggested by the attack's structure is to deny predictions on classes that have been forgotten, or to require that request sets contain no held-out images from those classes; the paper does not explore defenses.
- Inference: The same agreement-selection mechanism should transfer to class-membership recall in other modalities, such as text or tabular data, whenever the label space is unchanged, but the paper only tests image classifiers.
- Inference: An untested confound is whether recovered accuracy comes from genuinely recalling forgotten labels or from the student learning the anchor distribution; a permutation test on anchor labels would separate the two.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proposes a Membership Recall Attack (MRA) framework that recovers the class memberships of forgotten samples from machine unlearning models (ULMs) without access to the original trained model. MRA uses the ULM as a noisy-label teacher in a teacher-student knowledge distillation setup, applies mixup augmentation, and then performs a confident membership recall step based on top-K agreement between the teacher and student models. The method is evaluated on four datasets (CIFAR-10, CIFAR-100, Pet-37, Flower-102) and eight unlearning methods, in both a closed-source (black-box ULM) and an open-source (white-box ULM) setting. The authors report substantial improvements in accuracy on the forgotten set after MRA, up to 0.997 D_f accuracy for SalUn on CIFAR-10 in the open-source case, and conclude that precise unlearning methods are particularly vulnerable to this recall attack.
Significance. If the ULM-specific interpretation is correct, this is a novel and practically relevant attack: it is the first to claim class-membership recovery from unlearned models without access to the original trained model. The paper has concrete strengths: a clearly described protocol, evaluation across eight MU methods and four datasets, separate closed- and open-source cases, an ablation study, and an anonymous code repository. However, the central privacy-leak claim is not yet established because the experimental design lacks an exact-unlearning control. Without a retrained-from-scratch baseline, the reported D_f accuracy gains may reflect class-level information that any model trained on the remaining data would retain, rather than residual memory of the forgotten samples in the ULM. The conclusions therefore hinge on a comparison that the paper does not perform.
major comments (3)
- [Section 5.1, Table 1; conclusion in Section 5.2 and Section 6] The experimental protocol sets D_p = D_ts ∪ D_f and constructs D_f as a 50% subsample of five classes, so D_r still contains the other half of the training images from those same classes. A model retrained from scratch on D_tr \ D_f would therefore retain class-level discriminative information and could label a large fraction of D_f correctly simply by class similarity. The reported D_f accuracies after MRA (e.g., 0.997 for SalUn on CIFAR-10 in Table 3) are interpreted as evidence that the ULM leaks the forgotten class memberships, but without an exact-unlearning control (a retrained or SISA model used as the teacher in the same MRA pipeline), the same numbers could be produced by any model trained on the remaining data. This control is necessary to support the paper's central claim 'precise forgetting, easy recalling' and the privacy-leak conclusion.
- [Section 5.1, Tables 2 and 3] All reported accuracies are single runs with no error bars, standard deviations, or number of seeds. Several deltas are small (e.g., ∆Acc = 0.005 for GA on CIFAR-10 D_ts in Table 2), and the claim that RCMs 'unexceptionally improve' accuracy across all settings cannot be assessed without variance estimates. The authors should report multiple seeds (at least three) and, ideally, statistical significance for the key comparisons.
- [Section 3.1 and Section 5.1, Eqs. (5)-(8)] The attack's operational requirement is under-specified: D_p is defined to include both the forgetting images and the test set D_ts, and the confident membership recall step uses top-K selection over the whole of X_p. This means the adversary needs non-forgotten samples from the same classes as the forgotten ones to bootstrap the prototypes. The paper does not state this auxiliary-data condition as part of the threat model, nor does it evaluate failure modes when D_p contains only the forgotten images or when unlearning removes whole classes from the label space. Without such an evaluation or a stated assumption, the practical scope of the attack is unclear.
minor comments (5)
- [Section 4.3, Eq. (7)] The text reads 'where Y^U_p , Y^U_p ∈ R^{N×C}' but the second matrix should be Y^S_p; please correct this typo.
- [Section 5.2, first paragraph] The sentence 'Table 2 show the accuracy (Acc) of prediction dataset D_p in terms of its subsets D_ts and D_ts respectively' should refer to D_ts and D_f; the same typo appears in the Table 2 caption.
- [Section 4.1, subsection heading] The heading 'Denosing Knowledge Distillation' should be 'Denoising Knowledge Distillation'.
- [Section 3.1] The formal definition allows X_u ⊆ X_f, but the experiments set X_u = X_f (D_p = D_ts ∪ D_f); please state this explicitly to avoid confusion between the general formulation and the evaluation setup.
- [Figures 7, 16-31] The confusion-matrix figures would be easier to read if the color scale were shared across all panels for a given dataset and configuration.
Circularity Check
No material circularity: RCM accuracy is measured against ground-truth labels, not against the pseudo-labels used for training; the few self-citations are non-load-bearing.
full rationale
The paper's derivation chain is: the ULM provides pseudo-labels on X_p (constructed as D_ts ∪ D_f), a student model (and optionally the ULM itself in the open-source case) is trained on confident-agreement subsets D_CF selected by Eqs. (5)-(8), and the resulting RCM is evaluated by accuracy on the true labels of D_f and D_ts. Nothing in Eqs. (1)-(12) assumes the target membership labels: Y_f never enters the training loop, and the reported D_f accuracy is an external comparison against ground truth rather than a comparison against the teacher's pseudo-labels. The open-source variant is self-training on the ULM's own confident predictions, but the final metric is true-label accuracy, so the observed improvement is an empirical finding, not an identity forced by construction. The self-citations ([Bao et al. 2023], [Miao et al. 2024], [Wu et al. 2024]) appear only as motivational or background references and are not load-bearing for the MRA derivation. The main validity weakness, namely the absence of a retrained-from-scratch or exact-unlearning control, is a threat-model and experimental-design concern: it bears on whether the recovered labels reflect residual memory in the ULM versus class-level information retained in D_r, but it does not make the derivation circular. No circular step can be quoted from the paper's equations or argument.
Assumptions & free parameters
free parameters (6)
- tau (Top-K ratio) =
0.60 or 0.05 (closed-source per dataset), 0.05 (open-source)
- alpha_x (mixup image beta) =
0.20
- alpha_y (mixup label beta) =
0.75
- gamma_l (Laplace smoothing) =
0.01
- gamma_s (label smoothing) =
0.05
- learning rates =
LR_S 1e-4/2e-4, LR_U 5e-5/1e-4
assumptions (4)
- domain assumption The forgetting classes remain in the model's label space, and the prediction set D_p contains non-forgotten samples from those classes
- domain assumption The ULM is accessible either as a black-box service or with released parameters
- standard math Mixup and confidence-based pseudo-label selection can bootstrap a student from a noisy teacher
- domain assumption The MU algorithms are correctly implemented as published
Cite this review
Pith. "Pith review of Recalling The Forgotten Class Memberships: Unlearned Models Can Be Noisy Labelers to Leak Privacy." pith.science (2026). https://pith.science/paper/N5JXMSEX
@misc{pith2026250619486,
author = {Pith},
title = {Pith review of: Recalling The Forgotten Class Memberships: Unlearned Models Can Be Noisy Labelers to Leak Privacy},
year = {2026},
howpublished = {\url{https://pith.science/paper/N5JXMSEX}},
note = {Machine review of arXiv:2506.19486}
}
read the original abstract
Machine Unlearning (MU) technology facilitates the removal of the influence of specific data instances from trained models on request. Despite rapid advancements in MU technology, its vulnerabilities are still underexplored, posing potential risks of privacy breaches through leaks of ostensibly unlearned information. Current limited research on MU attacks requires access to original models containing privacy data, which violates the critical privacy-preserving objective of MU. To address this gap, we initiate an innovative study on recalling the forgotten class memberships from unlearned models (ULMs) without requiring access to the original one. Specifically, we implement a Membership Recall Attack (MRA) framework with a teacher-student knowledge distillation architecture, where ULMs serve as noisy labelers to transfer knowledge to student models. Then, it is translated into a Learning with Noisy Labels (LNL) problem for inferring the correct labels of the forgetting instances. Extensive experiments on state-of-the-art MU methods with multiple real datasets demonstrate that the proposed MRA strategy exhibits high efficacy in recovering class memberships of unlearned instances. As a result, our study and evaluation have established a benchmark for future research on MU vulnerabilities.
Figures
Figures from the paper (28 more)
Reference graph
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