Graph Neural Networks (GNNs) have achieved remarkable success in various applications, while still exhibiting high vulnerability to backdoor attacks when applied to node classification. Existing single-category attack methods typically rely on adaptive triggers that force victim nodes to be misclassified into a fixed target label, but they often neglect the inherent structural and feature priors associated with the target category. In this work, we propose a novel and effective backdoor attack framework Category-Specific Trigger Backdoor Attacks (CSTBA), employing category-specific information to generate more natural and unnoticeable triggers. Specifically, we introduce a Category-Specific Subgraph Triggers Pool (CS-STP) to capture representative patterns of the target category, along with a Match-and-Attach Strategy (MAS) to unnoticeably attach triggers to victim nodes, thereby ensuring that the modifications remain effective and unnoticeable within the graph. Extensive experiments on multiple benchmark datasets demonstrate that our method significantly enhances both the attack success rate (ASRs) and the unnoticeability of the attack compared with existing single-category approaches, highlighting the critical importance of category-aware trigger design in GNN backdoor attacks.

Graph Neural Networks (GNNs) have achieved significant success in various real-world applications, including social networks, finance systems, and traffic management. Recent researches highlight their vulnerability to backdoor attacks in node classification, where GNNs trained on a poisoned graph misclassify a test node only when specific triggers are attached. These studies typically focus on single attack categories and use adaptive trigger generators to create node-specific triggers. However, adaptive trigger generators typically have a simple structure, limited parameters, and lack category-aware graph knowledge, which makes them struggle to handle backdoor attacks across multiple categories as the number of target categories increases. We address this gap by proposing a novel approach for Effective and Unnoticeable Multi-Category (EUMC) graph backdoor attacks, leveraging subgraph from the attacked graph as category-aware triggers to precisely control the target category. To ensure the effectiveness of our method, we construct a Multi-Category Subgraph Triggers Pool (MC-STP) using the subgraphs of the attacked graph as triggers. We then exploit the attachment probability shifts of each subgraph trigger as category-aware priors for target category determination. Moreover, we develop a ``select then attach'' strategy that connects suitable category-aware trigger to attacked nodes for unnoticeability. Extensive experiments across different real-world datasets confirm the efficacy of our method in conducting multi-category graph backdoor attacks on various GNN models and defense strategies.