arXiv:2610.01263v2 Announce Type: replace Abstract: Open-source software (OSS) ecosystems face escalating supply chain threats, including typosquatting, dependency confusion, Trojan Source obfuscation, malicious build injection, and CI/CD pipeline poisoning. Sonatype documented over 1.2 million cumulative malicious packages across major registries, while signature-based and rule-based detection approaches lack the semantic understanding needed to generalize across evolving attacks. This paper proposes a taxonomy-aligned large language model framework for automated detection and multi-class classification of OSS supply chain threats. We introduce an AV-xxx taxonomy covering five semantically distinct attack categories and curate 999 verified real-world incidents from GitHub Security Advisories, CISA alerts, government security databases, and commercial threat intelligence reports, spanning 2018 to 2026 across eight package ecosystems. Using taxonomy-aligned prompting with GPT-4, the framework achieves 97% accuracy and 97% macro F1 across all five categories. Evaluation against eight methods five traditional machine learning baselines, one zero-shot open-source LLM, and two fine-tuned neural models reveals that fine-tuned Llama 3.1 8B (70.5%) and SecRoBERTa (77.5%) underperform a simple TF-IDF classifier (82.3%), while Mistral 7B without taxonomy alignment achieves 65.7%. These findings highlight taxonomy-aligned prompting over model scale, domain pretraining, or fine-tuning as the critical performance factor. Cross-ecosystem analysis shows varying classification difficulty, with npm achieving 65.5% baseline accuracy versus 100% in smaller ecosystems. GitHub Actions exhibits a substantial accuracy F1 divergence (82.4% accuracy, 0.367 macro F1), indicating systematic class imbalance and highlighting ecosystems where LLM-based contextual reasoning offers the greatest benefit. Our dataset, scripts, and code are publicly available in Github.