Capability analysis is a key building block for JavaScript supply-chain security tasks such as malware detection, security auditing, and runtime policy construction. However, existing approaches do not provide guarantees in the presence of JavaScript’s dynamic features.

We present a defensive capability analysis for JavaScript libraries that soundly reports every exercised capability for code executed under a lightweight protected runtime. The analysis tracks only objects that are relevant to capabilities. It reports capability usage either at the program locations where such objects are used, or conservatively when relevant objects escape through operations that the analysis does not model precisely. To address a small number of dynamic language features that would otherwise invalidate this analysis, we complement the static analysis with a lightweight runtime enforcement mechanism that blocks those patterns.

We implement the approach and evaluate it on large datasets of benign and malicious npm packages. The results show that the approach scales to real-world packages, pinpoints capability-usage locations with high accuracy, and remains compatible with 98.2% of benign libraries when using the protected runtime. Moreover, it detects capability usage in 99.8% of the malicious packages in our dataset, while the remaining cases are blocked by the runtime protection. Our study also shows that at least 72.9% of the npm packages use no security-sensitive capabilities at all, suggesting that capability analysis can substantially reduce auditing effort in practice.