Detecting Malicious NPM Packages

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.
• Vetting a full package.json / package-lock.json during code review or onboarding a third-party library.
• Responding to a supply-chain advisory (e.g., a worm campaign) and needing to check whether your lockfiles pulled a known-bad version.
• Investigating an endpoint or CI runner suspected of having installed a trojanized package.
• Building a pre-install gate in CI/CD that blocks packages exhibiting malicious indicators.

Prerequisites

• An isolated VM or disposable container with no production credentials and snapshot/rollback capability.
• GuardDog:

  pip install guarddog
  # or run via Docker without local install:
  docker pull ghcr.io/datadog/guarddog
  alias guarddog='docker run --rm ghcr.io/datadog/guarddog'
  

• Node.js + npm (use --ignore-scripts when downloading for analysis).
• jq, tar, and optionally OSV-Scanner for known-vulnerability/known-malicious cross-checks:

  go install github.com/google/osv-scanner/cmd/osv-scanner@v1
  

• For dynamic analysis: a sandbox with egress logging (e.g., tcpdump, a DNS sink, or a network namespace).

Objectives

• Statically scan an npm package (or a whole dependency tree) for malicious heuristics without executing it.
• Identify install-script abuse, environment/credential exfiltration, obfuscation, and silent process execution.
• Cross-check lockfile-pinned versions against known-malicious version lists / OSV.
• Safely detonate a suspect package and observe network and filesystem behavior.
• Extract indicators of compromise (URLs, IPs, hashes) for blocking and threat intel.
• Produce a defensible verdict (benign / suspicious / malicious) with evidence.

MITRE ATT&CK Mapping

Workflow

1. Download the package without executing it

Fetch the tarball with scripts disabled so nothing runs during acquisition.

mkdir triage && cd triage
# Resolve the tarball URL and download it (no install, no scripts)
npm pack express@4.18.2            # produces express-4.18.2.tgz
# or for an arbitrary version:
npm view some-pkg@1.2.3 dist.tarball
curl -sL "$(npm view some-pkg@1.2.3 dist.tarball)" -o some-pkg.tgz
tar -xzf some-pkg.tgz              # extracts into ./package

2. Scan a single package with GuardDog

GuardDog applies metadata + source heuristics and prints which rules matched.

# Scan the latest published version from the registry
guarddog npm scan express

# Scan a specific version
guarddog npm scan some-pkg --version 1.2.3

# Scan the local tarball / extracted directory you downloaded above
guarddog npm scan ./some-pkg.tgz
guarddog npm scan ./package/

3. Verify an entire dependency tree

verify scans every dependency declared in a manifest — ideal for code review.

guarddog npm verify /path/to/repo/package.json

4. Focus on the highest-signal heuristics

Filter to the npm rules most indicative of malware to cut noise during triage.

guarddog npm scan some-pkg \
  --rules npm-install-script \
  --rules npm-serialize-environment \
  --rules npm-exec-base64 \
  --rules npm-silent-process-execution \
  --rules npm-obfuscation \
  --rules shady-links \
  --rules typosquatting

5. Emit machine-readable output for pipelines

JSON for tooling, SARIF for GitHub code scanning.

guarddog npm scan some-pkg --output-format=json   > guarddog.json
guarddog npm verify package.json --output-format=sarif > guarddog.sarif

6. Manually inspect lifecycle scripts and source

Lifecycle scripts are the first thing to read; obfuscation and outbound URLs are red flags.

# Show all lifecycle hooks
jq '.scripts' package/package.json

# Hunt for exfiltration / execution primitives in the source
grep -rEn "child_process|exec\(|spawn|eval\(|Buffer\.from\(.*base64|process\.env|https?://" package/ \
  --include='*.js' --include='*.ts' | head -50

7. Cross-check lockfiles against known-malicious versions

During an active campaign, compare pinned versions to the advisory's bad-version list, and run OSV.

# Extract resolved name@version pairs from a v3 lockfile
jq -r '.packages | to_entries[] | select(.key|startswith("node_modules/")) | "\(.key|ltrimstr("node_modules/"))@\(.value.version)"' package-lock.json

# OSV-Scanner flags known-vulnerable AND known-malicious (MAL-) advisories
osv-scanner --lockfile=package-lock.json

8. Detonate safely with monitoring (only if static is inconclusive)

Run the install inside a disposable, network-monitored sandbox.

# In a throwaway container / VM with egress capture running (tcpdump -w capture.pcap):
npm install ./some-pkg.tgz            # scripts WILL run — sandbox only
# Baseline-diff the filesystem afterwards for writes outside node_modules,
# and inspect capture.pcap for unexpected DNS / HTTP beacons.

9. Extract and operationalize IOCs

Pull URLs, IPs, and hashes for blocking and intel sharing.

grep -rhoE "https?://[a-zA-Z0-9./?=_%:-]+" package/ | sort -u > urls.txt
sha256sum some-pkg.tgz package/*.js > hashes.txt

10. Run the bundled triage helper

agent.py orchestrates GuardDog, lifecycle-script inspection, and IOC extraction into one report.

python scripts/agent.py --package some-pkg --version 1.2.3 --output verdict.json
# or against a local tarball:
python scripts/agent.py --tarball ./some-pkg.tgz --output verdict.json

Tools and Resources

Validation Criteria

• Package acquired with scripts disabled in an isolated environment.
• GuardDog scan run on the target version with results captured.
• Full dependency tree run through GuardDog verify where applicable.
• Lifecycle scripts (preinstall/install/postinstall) read and assessed.
• Lockfile versions cross-checked against OSV / known-bad lists.
• Dynamic detonation performed in a sandbox if static analysis was inconclusive.
• IOCs (URLs, IPs, hashes) extracted and recorded.
• Documented verdict (benign / suspicious / malicious) with supporting evidence.
• Malicious findings reported to the registry and shared as threat intel.

Source: mukul975/Anthropic-Cybersecurity-Skills → skills/detecting-malicious-npm-packages/SKILL.md