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detecting-malicious-npm-packages

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Triage npm packages and lockfiles for install-script malware, credential exfiltration, and worming behavior using GuardDog, manual tarball inspection, and dynamic detonation with network/filesystem monitoring. Use when vetting a new dependency, reviewing a package.json/package-lock.json during code review, checking lockfiles against a supply-chain advisory's known-bad versions, or investigating a host suspected of installing a trojanized package.

skills/detecting-malicious-npm-packages/SKILL.md

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Network or broad shell
Last checked
Nov 9, 2026

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Repository last updated
Aug 31, 2026
License
Apache-2.0

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# Detecting Malicious npm Packages

> **Legal Notice:** Analyze packages in an isolated, disposable environment. Some malicious packages execute on install (`npm install` runs lifecycle scripts automatically) or on import. Never analyze a suspect package on a workstation with credentials, SSH keys, cloud tokens, or network access to production. This skill is for defensive analysis and authorized incident response only.

## Overview

The npm registry is the largest software package ecosystem in the world and the most heavily targeted by supply-chain attackers. Malicious packages reach victims through typosquatting (`expresss`, `crossenv`), dependency confusion, account/maintainer takeover (the 2025 Shai-Hulud worm and the `event-stream` compromise are canonical examples), and starjacking. The defining danger of npm is that `npm install` automatically runs `preinstall`, `install`, and `postinstall` lifecycle scripts with the developer's full privileges **before any application code is invoked** — so simply installing a package is enough to be compromised. Roughly 2% of npm packages use install scripts, which makes them both common and a powerful malware delivery vehicle.

Typical malicious behaviors are: exfiltrating environment variables, `~/.npmrc` tokens, SSH keys, and cloud credentials to an attacker-controlled URL; opening reverse shells; dropping cryptominers; reading and posting `process.env`; obfuscating payloads with base64/eval; and self-propagating (worming) by stealing the maintainer's npm token and republishing trojanized versions of other packages they own.

This skill provides a repeatable triage workflow centered on **GuardDog** (Datadog's open-source heuristic scanner built on Semgrep + metadata rules), supplemented by manual tarball inspection, lockfile-based compromise checks against known-bad version lists, and dynamic detonation with network and filesystem monitoring. The goal is to decide, quickly and safely, whether a given package or a project's dependency tree contains malicious code.

## When to Use

- Triaging a specific npm package before adding it as a dependency.
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