Malicious URL detection remains a critical cybersecurity challenge as adversaries increasingly employ sophisticated evasion techniques including obfuscation, character-level perturbations, and adversarial attacks. Although pre-trained language models (PLMs) like BERT have shown potential for URL analysis tasks, three limitations persist in current implementations: (1) inability to effectively model the non-natural hierarchical structure of URLs, (2) insufficient sensitivity to character-level obfuscation, and (3) lack of mechanisms to incorporate auxiliary network-level signals such as IP addresses-all essential for robust detection. To address these challenges, we propose CURL-IP, an advanced multi-modal detection framework incorporating three key innovations: (1) Token-Contrastive Representation Enhancer, which enhances subword token representations through token-aware contrastive learning to produce more discriminative and isotropic embeddings; (2) Cross-Layer Multi-Scale Aggregator, employing hierarchical aggregation of Transformer outputs via convolutional operations and gated MLPs to capture both local and global semantic patterns across layers; and (3) Blockwise Multi-Modal Coupler that decomposes URL-IP features into localized block units and computes cross-modal attention weights at the block level, enabling fine-grained inter-modal interaction. This architecture enables simultaneous preservation of fine-grained lexical cues, contextual semantics, and integration of network-level signals. Our evaluation on large-scale real-world datasets shows the framework significantly outperforms state-of-the-art baselines across binary and multi-class classification tasks.

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