Multi-Patch Grid Attack: A Distributed and Defense-Resilient Backdoor in Federated Medical Attention Models

Vision Transformers (ViTs) are increasingly being adopted in federated medical imaging; however, their patch-based attention architecture introduces unexplored backdoor vul- nerabilities. Existing attacks target convolutional neural net- work (CNN) architectures with localized triggers and are readily mitigated by norm-based and clustering defenses, leaving federated ViT systems under-evaluated. We pro- pose the multi-patch grid attack (MPGA), a distributed back- door that exploits ViT patch tokenization by placing syn- chronized perturbations across four corner patches (8×8 pixels, intensity 0.9) and a central cross pattern (1-pixel width, intensity 0.6), distributing malicious features across multi- ple attention tokens. This design maintains gradient sim- ilarity to benign updates (cosine similarity 0.89, norm ra- tio 0.944±0.08) and pixel-level distribution divergence be- low DK L =0.0123. Evaluated on the Figshare brain tumor MRI dataset across 10 federated clients, MPGA achieves 94.13% backdoor success rate (BSR) of 94.13 %, with 86.13% main task accuracy (MTA) preserved, and generalizes to the IEEE Dataport dataset (95.00% BSR, 89.33% MTA). The back- door persists after attack cessation, retaining 87.00% ASR after 15 consecutive benign-only rounds. Against ten state- of-the-art defenses spanning Byzantine-robust aggregation, Sybil detection, behavioral monitoring, and trigger-agnostic inversion (Neural Cleanse, ABS, STRIP), no defense meets the combined success criteria (ASR<40%, MTA>85%, F1>0.70). Compared with BadNets and DBA, MPGA achieves the high- est ASR across all evaluated defenses, confirming that ViT patch-boundary alignment is the key differentiator. These results expose fundamental limitations of current defense paradigms against architecture-aware distributed backdoors in federated ViT systems.