slopcheck

An LLM wrote npx react-codeshift across 47 files. The package didn't exist. Then someone registered it. 237 repos had already referenced it.

slopcheck validates npm package names in your docs and config files against the live registry — before that someone is waiting.

Quickstart

npx slopcheck .

GitHub Action

name: Slopsquatting Check
on: [push, pull_request]
jobs:
  slopcheck:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: mattschaller/slopcheck@v0.2.0
        with:
          paths: '.'

Findings appear as workflow annotations directly on the PR diff.

The problem

AI coding agents hallucinate package names. According to USENIX Security 2025 research, ~20% of AI-generated code references packages that don't exist on npm. 58% of these hallucinated names recur consistently across prompts, making them predictable squat targets. Attackers register these phantom names as malware and wait for installs.

Your AGENTS.md, SKILL.md, .cursorrules, .mdc, and documentation files are attack surfaces.

Why this matters for AI agent skills

ClawHub's official skills registry had 1,184 confirmed malicious entries in 2026 (Antiy CERT). The #1 most-downloaded skill was a macOS malware stager. Many attacks work by referencing a plausible-looking npm package that doesn't exist — yet.

slopcheck catches those references before your agents install them.

How it works

Scans markdown (.md, .mdc), YAML, and JSON files for install commands (npm install, npx, pnpm add, yarn add, bun add, bunx) → extracts package names → validates each against the live npm registry → reports phantom packages.

Zero runtime dependencies. Uses only Node.js built-in APIs.

How it's different from Snyk and Socket

Snyk and Socket evaluate packages that exist on npm. slopcheck catches references to packages that don't exist yet — the gap before any advisory database can know about them.

What it catches

Attack vector	How it works	slopcheck defense
Hallucinated package in AGENTS.md	AI generates npx react-codeshift, attacker registers the name	Detects react-codeshift doesn't exist on npm
Hallucinated dependency in README	Developer copies npm install phantom-pkg from docs	Flags phantom-pkg as not found
AI-generated SKILL.md with phantom packages	Spread across 237+ repos via copy/paste	Catches all non-existent package references
Unpublished packages	Package was removed from npm but had downloads (takeover risk)	Detects via npm downloads API, flags as unpublished
Security-held packages	Package removed by npm for malware (HTTP 451)	Flags with security hold warning

Example output

slopcheck v0.2.0 — scanning 3 files for phantom packages

✗ left-pad — unpublished from npm (takeover risk)
  └─ AGENTS.md:22  npm install left-pad

✗ react-codeshift — not found on npm
  └─ AGENTS.md:14  npx react-codeshift --transform ...
  └─ SKILL.md:8    npm install react-codeshift

⚠ suspicious-pkg — security hold (HTTP 451)
  └─ .cursorrules:19  npm install suspicious-pkg

✓ 12 packages verified, 1 unpublished, 1 not found, 1 security hold

Found 2 phantom packages. Exit code 1.

CLI options

Usage: slopcheck [options] [files/directories...]

Options:
  -V, --version        output version
  --json               output JSON instead of text
  --concurrency <n>    max concurrent registry checks (default: 10)
  --ignore <packages>  comma-separated list of packages to skip
  --no-security-hold   don't flag security holds as warnings
  -h, --help           display help

Arguments:
  files/directories    files or directories to scan (default: current directory)
                       directories are scanned recursively for .md, .mdc, .yml, .yaml,
                       .json, .cursorrules files
                       node_modules, .git, dist, build directories are always excluded

GitHub Action options

- uses: mattschaller/slopcheck@v0.2.0
  with:
    paths: '. docs/'
    ignore: 'my-internal-pkg,another-known-pkg'
    concurrency: '10'

What it doesn't catch

Honesty builds trust:

• Doesn't scan package.json or lock files. Use Socket.dev or Snyk for dependency analysis.
• Doesn't check if an existing package is malicious. Use @aikidosec/safe-chain for runtime install protection.
• Only checks if the package name exists on npm. A package existing doesn't mean it's safe — it means it's not a hallucination.

Prior art and references

• USENIX Security 2025 — "We Have a Package for You!" — 576K code samples, 16 models, ~20% hallucinated package names
• Aikido Blog — Slopsquatting — attack vector analysis
• Aikido Blog — Agent Skills Spreading Hallucinated npx Commands — wshobson/agents incident, 237 repos, npx react-codeshift
• Antiy CERT — ClawHavoc Campaign — 1,184 malicious skills on ClawHub
• HackerOne — Slopsquatting: AI and Supply Chain Security — industry impact
• Trend Micro — Slopsquatting: When AI Agents Hallucinate Malicious Packages — supply chain attack patterns

Roadmap

• v0.3.0 — SARIF output for GitHub Code Scanning integration

Contributing

See CONTRIBUTING.md.

License

MIT