PatchCon: Patch-aware Weighted Supervised Contrastive Learning for Vulnerability Detection

Automated vulnerability detection is critical for securing modern software systems, yet existing learning-based approaches often treat functions as independent samples and underuse the supervision provided by vulnerability-fixing patches. In this paper, we propose PatchCon, a CodeBERT-based patch-aware contrastive learning model for function-level vulnerability detection. PatchCon jointly optimizes vulnerability classification and supervised contrastive learning, and treats vulnerable--patched function pairs as hard negatives with larger contrastive weights to capture subtle security-relevant differences introduced by fixes. To evaluate patch-level discrimination, we use two pairwise metrics that measure whether a model can correctly distinguish vulnerable functions from their patched counterparts. We evaluate PatchCon on PairVul-Linux through extensive experiments, including state-of-the-art comparisons, ablation studies, hyperparameter sensitivity analysis, and statistical significance testing. We also conduct fine-grained analyses by Common Weakness Enumeration (CWE) type, function length, and patch size, and further evaluate PatchCon on Linux kernel vulnerabilities disclosed after the training data cutoff. On PairVul-Linux, PatchCon outperforms representative pretrained code models and contrastive learning baselines on both classification and pairwise metrics, improves pairwise accuracy over the strongest reproduced baseline from 0.2060 to 0.3264, and achieves pairwise accuracy comparable to that of a GPT-4o-based large language model (LLM) method at a substantially lower cost. Among the reproduced models, PatchCon also achieves the highest pairwise accuracy on test pairs from Common Vulnerabilities and Exposures (CVE) entries unseen during training and on newly disclosed vulnerabilities.