Vertical multimodal federated learning (VMFL) enables multiple clients holding data from different modalities to conduct collaboratively model training. Existing methods typically assume that multimodal data samples (i.e., text and image) from the same entity (i.e., person) are paired across the clients (i.e., aligned). However, this assumption rarely holds in practice, as data is often collected independently with no shared identifiers. To address this challenge, we propose hashing-based alignment (HAL), a new VMFL framework that works without pre-aligned samples. HAL consists of two key components. The first component is an efficient and privacy-preserving method to identify similar samples from different modalities as aligned pairs. It adopts locality sensitive hashing (LSH) for the efficient retrieval of similar samples, introduces a shift-orthogonal hashing scheme to tackle the gaps between different modalities, and uses a bloom-style method for secure Hamming distance estimation. We prove that the shift-orthogonal hashing reduces distance estimation errors and secure Hamming distance estimation satisfies differential privacy. The second component is a neighbor-aware fusion strategy, which applies cross-attention to aggregate informative signals from the aligned samples without relying on explicit similarity scores. Experimental results on two real-world datasets show that compared with five state-of-the-art (SOTA) baselines, HAL improves the cross-modal retrieval accuracy by over 63%, while also achieving up to 154× speedup.

Attribute Inference Attacks (AIAs) pose a significant threat to recommendation systems (RS) by enabling adversaries to use threat models to infer sensitive user attributes like gender or race from user embeddings, resulting in privacy breaches such as unauthorized profiling and discriminatory policies against specific groups. Existing attribute protection methods are primarily applied during training, suffering from significant limitations, such as architectural inflexibility, dependence on interaction data, and potential catastrophic degradation in recommendation performance. To overcome these challenges, we propose AttrCloak, an efficient and effective post-training attribute unlearning (AU) framework that removes sensitive information from user embeddings without altering RS training architectures. AttrCloak employs dual-objective optimization with parameter self-sharing to minimize mutual information between user embeddings and sensitive attributes while preserving recommendation quality. Furthermore, it accommodates data-free scenarios by leveraging regularization loss when interaction data is unavailable. Comprehensive evaluations on four real-world datasets demonstrate AttrCloak's good performance in privacy protection and recommendation performance.

As privacy concerns in recommender systems become increasingly prominent, federated recommender systems (FedRecs) have emerged as a promising distributed training paradigm.FedRecs enable the collaborative training of a shared global recommendation model without requiring the exchange of raw client interaction data.However, models trained using standard FedRec methods remain vulnerable to personal information leakage, particularly through attribute inference attacks, which can expose sensitive user attributes such as gender and race.In this paper, we address these user-sensitive attributes as targets for federated unlearning.To protect users' sensitive information, attribute unlearning aims to eliminate sensitive attributes from user embeddings, thereby preventing inference attacks while preserving recommendation performance.We introduce a novel post-training federated unlearning framework, Aegis, which performs unlearning based on private attribute requests after the model has been trained, minimizing the degradation in recommendation accuracy.Aegis employs an information-theoretic multi-component loss function to balance privacy protection and recommendation performance.Additionally, Aegis adapts to scenarios where training interaction data may be unavailable, reflecting real-world centralized protection scenarios.Comprehensive evaluations of various benchmark datasets demonstrate that our proposed method effectively safeguards user privacy while maintaining high-quality recommendations. CCS Concepts• Information systems → Social recommendation

For graph self-supervised learning (GSSL), masked autoencoder (MAE) follows the generative paradigm and learns to reconstruct masked graph edges or node features while contrastive learning (CL) maximizes the similarity between augmented views of the same graph. Existing works utilize MAE and CL separately but we observe that the MAE and CL paradigms are complementary and propose the graph contrastive masked autoencoder (GCMAE) framework to unify them. Specifically, by focusing on local edges or node features, MAE cannot capture global information of the graph and is sensitive to particular edges and features. On the contrary, CL excels in extracting global information because it considers the relation between graphs. As such, we equip GCMAE with an MAE branch and a CL branch, and the two branches share a common encoder, which allows the MAE branch to exploit the global information extracted by the CL branch. To force GCMAE to capture global graph structures, we train it to reconstruct the entire adjacency matrix instead of only the masked edges as in existing works. Moreover, a discrimination loss is proposed for feature reconstruction, which improves the disparity between node embeddings rather than reducing the reconstruction error to tackle the feature smoothing problem of MAE. We evaluate GCMAE on four popular graph tasks (i.e., node classification, node clustering, link prediction, and graph classification) and compare it with 14 state-of-the-art baselines. The results show that GCMAE consistently provides good accuracy across these tasks, and the maximum accuracy improvement is up to 3.2% compared with the best-performing baseline.

Nowadays, the surge of Internet contents and the need for high Quality of Experience (QoE) put the backbone network under unprecedented pressure. The emerging edge caching solutions help ease the pressure by caching contents closer to users. However, these solutions suffer from two challenges: 1) a low hit ratio due to edges’ high density and small coverages. 2) unbalanced edges’ workloads caused by dynamic requests and heterogeneous edge capacities. In this paper, we formulate a typical cooperative edge caching problem and propose the MagNet, a decentralized and cooperative edge caching system to address these two challenges. The proposed MagNet system consists of two innovative mechanisms: 1) the Automatic Content Congregating (ACC), which utilizes a neural embedding algorithm to capture underlying patterns of historical traces to cluster contents into some types. The ACC then can guide requests to their optimal edges according to their types so that contents congregate automatically in different edges by type. This process forms a virtuous cycle between edges and requests, driving a high hit ratio. 2) the Mutual Assistance Group (MAG), which lets idle edges share overloaded edges’ workloads by forming temporary groups promptly. To evaluate the performance of MagNet, we conduct experiments to compare it with classical, Machine Learning (ML)-based and cooperative caching solutions using the real-world trace. The results show that the MagNet can improve the hit ratio from 40% and 60% to 75% for non-cooperative and cooperative solutions, respectively, and significantly improve the balance of edges’ workloads.