VulExplainer: A Transformer-Based Hierarchical Distillation for Explaining Vulnerability Types

Deep learning-based vulnerability prediction approaches are proposed to help under-resourced security practitioners to detect vulnerable functions. However, security practitioners still do not know what type of vulnerabilities correspond to a given prediction (aka CWE-ID). Thus, a novel approach to explain the type of vulnerabilities for a given prediction is imperative. In this paper, we propose VulExplainer, an approach to explain the type of vulnerabilities. We represent VulExplainer as a vulnerability classification task. However, vulnerabilities have diverse characteristics (i.e., CWE-IDs) and the number of labeled samples in each CWE-ID is highly imbalanced (known as a highly imbalanced multi-class classification problem), which often lead to inaccurate predictions. Thus, we introduce a Transformer-based hierarchical distillation for software vulnerability classification in order to address the highly imbalanced types of software vulnerabilities. Specifically, we split a complex label distribution into sub-distributions based on CWE abstract types (i.e., categorizations that group similar CWE-IDs). Thus, similar CWE-IDs can be grouped and each group will have a more balanced label distribution. We learn TextCNN teachers on each of the simplified distributions respectively, however, they only perform well in their group. Thus, we build a transformer student model to generalize the performance of TextCNN teachers through our hierarchical knowledge distillation framework. Through an extensive evaluation using the real-world 8,636 vulnerabilities, our approach outperforms all of the baselines by 5%-29%. The results also demonstrate that our approach can be applied to Transformer-based architectures such as CodeBERT, GraphCodeBERT, and CodeGPT. Moreover, our method maintains compatibility with any Transformer-based model without requiring any architectural modifications but only adds a special distillation token to the input. These results highlight our significant contributions towards the fundamental and practical problem of explaining software vulnerability.