Checking License Compliance

Checking License Compliance

Overview

License compliance is a security concern only in the indirect sense that an unintended license obligation can force you to release proprietary source code, retroactively invalidate a customer contract, or render an M&A transaction infeasible. The cost is legal and contractual rather than exploitative — but the consequence ladder is real.

The most-stepped-on landmine is copyleft contamination: unintentionally including a GPL or AGPL-licensed package in a codebase the rest of which is permissively licensed (MIT, Apache-2.0, BSD). The terms of the GPL family say that any project distributing GPL code MUST itself release source under a GPL-compatible license. If your package.json says MIT and one of your transitive deps is GPL-2.0, you may be obligated to either re-license your code or remove the dep.

This skill audits the resolved dependency tree against an explicit policy file and emits findings for:

• Direct deps with deny-listed licenses
• Transitive deps with deny-listed licenses
• Packages with UNKNOWN license metadata (no SPDX identifier)
• License conflicts between metadata and source headers
• Combinations of licenses that are mutually incompatible (e.g. GPL-2.0 + Apache-2.0 without a patent grant)

When the skill produces findings

Prerequisites

• Python 3.9+
• Target project with EITHER a package.json + node_modules/ OR a Python project (pyproject.toml/requirements.txt/ installed venv)
• Policy file at ./.license-policy.json (auto-detected) or passed via --policy. If absent, the skill uses a built-in default policy that flags strong copyleft for permissive parent projects.

Instructions

Step 1 — Identify the project's own declared license

The skill reads the project's top-level license from:

• npm: package.json's license field
• Python: pyproject.toml's [project].license table OR setup.cfg's license field

If the project's own license isn't declared, the skill emits a FATAL operational finding — license compliance can't be checked without a baseline. Add a license field before running.

Step 2 — Identify policy

The policy file is JSON:

{
  "allow": ["MIT", "BSD-3-Clause", "Apache-2.0", "ISC", "BSD-2-Clause"],
  "deny":  ["GPL-2.0-only", "GPL-3.0-only", "AGPL-3.0-only", "AGPL-3.0-or-later"],
  "review": ["MPL-2.0", "EPL-2.0", "CDDL-1.0", "LGPL-3.0-or-later"],
  "project_license": "MIT"
}

allow: licenses that pass without comment. deny: licenses that produce a finding regardless of project license. review: licenses that produce a MEDIUM-severity finding for legal review. project_license: enforced — if the project declares this but a dep is in deny, finding is CRITICAL.

Step 3 — Run the scanner

python3 ./scripts/check_licenses.py /path/to/project

Options:

Usage: check_licenses.py PATH [OPTIONS]

Options:
  --output FILE      Write findings to FILE (default: stdout)
  --format FMT       json | jsonl | markdown (default: markdown)
  --min-severity SEV (default: info)
  --policy FILE      Override default policy
  --emit-attribution  Also emit an attribution file (NOTICE.md) listing
                     every permissive-licensed dep that requires attribution

Step 4 — Interpret findings

CRITICAL findings block release pending legal review. Either remove the offending dep, replace it with a permissively-licensed alternative, or escalate to legal for a written exception.

HIGH findings require legal sign-off but don't necessarily block release if the legal posture (e.g. service-only deployment under AGPL) makes the obligation moot.

MEDIUM findings should be reviewed quarterly and either resolved or moved into an explicit exception list.

INFO findings are reminders that an attribution / NOTICE file should reference these packages.

Examples

Example 1 — Pre-release legal gate

python3 ./scripts/check_licenses.py . --min-severity high --format json --output license-audit.json
jq -e '. == []' license-audit.json || { echo "License finding — legal review required"; exit 1; }

Example 2 — Generate attribution file

python3 ./scripts/check_licenses.py . --emit-attribution --format markdown --output NOTICE.md

Example 3 — M&A due diligence

mkdir -p evidence/legal/
python3 ./scripts/check_licenses.py target-acquisition-codebase/ \
    --format json \
    --output evidence/legal/license-audit-$(date +%Y%m%d).json

Output

JSON / JSONL / Markdown per lib/report.py. Exit codes: 0 clean, 1 high/critical, 2 error.

Each Finding includes:

• id — license-compliance::<package>::<license-id>
• severity — CRITICAL / HIGH / MEDIUM / LOW / INFO
• category — license-compliance
• summary — what's wrong
• evidence — package name, declared license, project license, policy match
• references — SPDX URL for the license, package home page

Error Handling

• No project license → emits an INFO/operational finding recommending the operator add a license field, exits 2.
• Unparseable policy file → exits 2 with a parser error message.
• Package with malformed license field → treated as UNKNOWN license, emits MEDIUM finding.
• No SPDX identifier in source headers → emits INFO finding reminding that SPDX header convention catches contamination at the file level.

Resources

• references/THEORY.md — SPDX license expression syntax, family classifications, copyleft propagation theory, common license incompatibilities, when LGPL static linking matters, AGPL service-distribution clauses, public-domain edge cases (CC0 vs unlicense)
• references/PLAYBOOK.md — Default policy templates per project type (proprietary product, OSS library, internal-only tool, SaaS service), attribution file generation, legal-counsel handoff templates, replacing copyleft deps with permissive alternatives

Source: jeremylongshore/claude-code-plugins-plus-skills → skills/.curated/checking-license-compliance/SKILL.md

Also appears in: jeremylongshore/claude-code-plugins-plus-skills/plugins/security/penetration-tester/skills/checking-license-compliance/SKILL.md