TGNN: Enhancing Pixel Tracking Detection via LLM-driven Annotation and GAT-powered Structural Representation

Web tracking is increasingly pervasive, raising serious concerns about user privacy and security. Among existing techniques, pixel tracking is particularly stealthy and cost-effective, embedding invisible images that exfiltrate user activities to third-party servers. Current defenses, including filter list blocking and conventional machine learning, often fail to capture the cross-site associations that enable pixel tracking to evade detection. To address this limitation, we introduce TGNN, a framework that formulates pixel tracking detection as an edge classification task on a Tracking Directed Graph (TDG), which models third-party associations across websites. TGNN encodes HTTP traffic into structured quadruples and learns both semantic features and interaction patterns. To overcome the scarcity of reliable labels, we propose a large language model (LLM)-based annotation method that leverages minimal expert supervision to produce high-quality labels, significantly improving detection. Experiments conducted on traffic from the Alexa top-10K websites demonstrate that TGNN substantially outperforms existing baselines, while the LLM-based annotation achieves accuracy comparable to expert curation. Our large-scale measurement reveals that at least 16.74% of websites engage in pixel tracking via major third-party infrastructures, establishing cross-domain tracking as a pervasive practice in the wild and indicating a potential privacy threat in the modern Web ecosystem.