Debiased Graph Poisoning Attack via Contrastive Surrogate Objective

TL;DR

This paper identifies bias in existing meta-gradient-based graph poisoning attacks towards training nodes and proposes a new contrastive surrogate objective method, Metacon, to improve attack effectiveness by reducing this bias.

Contribution

The paper introduces Metacon, a novel meta-gradient-based attack method that mitigates bias towards training nodes using a contrastive surrogate objective, enhancing attack performance.

Findings

Metacon outperforms existing meta-gradient-based attacks on benchmark datasets.

Alleviating bias towards training nodes improves attack effectiveness.

The contrastive surrogate objective effectively reduces attack bias.

Abstract

Graph neural networks (GNN) are vulnerable to adversarial attacks, which aim to degrade the performance of GNNs through imperceptible changes on the graph. However, we find that in fact the prevalent meta-gradient-based attacks, which utilizes the gradient of the loss w.r.t the adjacency matrix, are biased towards training nodes. That is, their meta-gradient is determined by a training procedure of the surrogate model, which is solely trained on the training nodes. This bias manifests as an uneven perturbation, connecting two nodes when at least one of them is a labeled node, i.e., training node, while it is unlikely to connect two unlabeled nodes. However, these biased attack approaches are sub-optimal as they do not consider flipping edges between two unlabeled nodes at all. This means that they miss the potential attacked edges between unlabeled nodes that significantly alter the representation of a node. In this paper, we investigate the meta-gradients to uncover the root cause of the uneven perturbations of existing attacks. Based on our analysis, we propose a Meta-gradient-based attack method using contrastive surrogate objective (Metacon), which alleviates the bias in meta-gradient using a new surrogate loss. We conduct extensive experiments to show that Metacon outperforms existing meta gradient-based attack methods through benchmark datasets, while showing that alleviating the bias towards training nodes is effective in attacking the graph structure.