container-hadolint

Dockerfile Security Linting with Hadolint

Overview

Hadolint is a Dockerfile linter that validates container build files against security best practices and the CIS Docker Benchmark. It analyzes Dockerfile instructions to identify misconfigurations, anti-patterns, and security vulnerabilities before images are built and deployed.

Hadolint integrates ShellCheck to validate RUN instructions, ensuring shell commands follow security best practices. With 100+ built-in rules mapped to CIS Docker Benchmark controls, Hadolint provides comprehensive security validation for container images.

Quick Start

Install Hadolint

# macOS via Homebrew
brew install hadolint

# Linux via binary
wget -O /usr/local/bin/hadolint https://github.com/hadolint/hadolint/releases/latest/download/hadolint-Linux-x86_64
chmod +x /usr/local/bin/hadolint

# Via Docker
docker pull hadolint/hadolint

Scan Dockerfile

# Scan Dockerfile in current directory
hadolint Dockerfile

# Scan with specific Dockerfile path
hadolint path/to/Dockerfile

# Using Docker
docker run --rm -i hadolint/hadolint < Dockerfile

Generate Report

# JSON output for automation
hadolint -f json Dockerfile > hadolint-report.json

# GitLab Code Quality format
hadolint -f gitlab_codeclimate Dockerfile > hadolint-codeclimate.json

# Checkstyle format for CI integration
hadolint -f checkstyle Dockerfile > hadolint-checkstyle.xml

Core Workflows

1. Local Development Scanning

Validate Dockerfiles during development:

# Basic scan with colored output
hadolint Dockerfile

# Scan with specific severity threshold
hadolint --failure-threshold error Dockerfile

# Show only warnings and errors
hadolint --no-color --format tty Dockerfile | grep -E "^(warning|error)"

# Verbose output with rule IDs
hadolint -t style -t warning -t error Dockerfile

Output Format:

Dockerfile:3 DL3008 warning: Pin versions in apt get install
Dockerfile:7 DL3025 error: Use JSON notation for CMD and ENTRYPOINT
Dockerfile:12 DL3059 info: Multiple RUN instructions detected

When to use: Developer workstation, pre-commit validation, iterative Dockerfile development.

2. CI/CD Pipeline Integration

Automate Dockerfile validation in build pipelines:

GitHub Actions

name: Hadolint
on: [push, pull_request]

jobs:
  lint:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v3

      - name: Hadolint Dockerfile
        uses: hadolint/hadolint-action@v3.1.0
        with:
          dockerfile: Dockerfile
          failure-threshold: warning
          format: sarif
          output-file: hadolint.sarif

      - name: Upload SARIF to GitHub Security
        if: always()
        uses: github/codeql-action/upload-sarif@v2
        with:
          sarif_file: hadolint.sarif

GitLab CI

hadolint:
  image: hadolint/hadolint:latest-debian
  stage: lint
  script:
    - hadolint -f gitlab_codeclimate Dockerfile > hadolint-report.json
  artifacts:
    reports:
      codequality: hadolint-report.json
    when: always

When to use: Automated security gates, pull request checks, deployment validation.

3. Configuration Customization

Create .hadolint.yaml to customize rules:

# .hadolint.yaml
failure-threshold: warning
ignored:
  - DL3008  # Allow unpinned apt-get packages (assess risk first)
  - DL3059  # Allow multiple RUN instructions

trustedRegistries:
  - docker.io/library  # Official Docker Hub images
  - gcr.io/distroless  # Google distroless images
  - registry.access.redhat.com  # Red Hat registry

override:
  error:
    - DL3001  # Enforce: never use yum/dnf/zypper without version pins
  warning:
    - DL3015  # Warn: use --no-install-recommends with apt-get
  info:
    - DL3059  # Info: multiple RUN instructions reduce layer caching

label-schema:
  maintainer: text
  org.opencontainers.image.vendor: text
  org.opencontainers.image.version: semver

Use bundled templates in assets/:

• assets/hadolint-strict.yaml - Strict security enforcement (CRITICAL/HIGH only)
• assets/hadolint-balanced.yaml - Balanced validation (recommended)
• assets/hadolint-permissive.yaml - Permissive for legacy Dockerfiles

When to use: Reducing false positives, organizational standards, legacy Dockerfile migration.

4. Security-Focused Validation

Enforce critical security rules:

# Only fail on security issues (error severity)
hadolint --failure-threshold error Dockerfile

# Check specific security rules
hadolint --trusted-registry docker.io/library Dockerfile

# Scan all Dockerfiles in project
find . -name "Dockerfile*" -exec hadolint {} \;

# Generate security report with only errors
hadolint -f json Dockerfile | jq '.[] | select(.level == "error")'

Critical Security Rules:

• DL3000: Use absolute WORKDIR (prevents directory traversal)
• DL3001: Always use version pinning for package managers
• DL3002: Never switch to root USER in Dockerfile
• DL3020: Use COPY instead of ADD (prevents arbitrary URL fetching)
• DL3025: Use JSON notation for CMD/ENTRYPOINT (prevents shell injection)

See references/security_rules.md for complete security rule catalog with CIS mappings.

5. Multi-Stage Build Validation

Scan complex multi-stage Dockerfiles:

# Validate all stages
hadolint Dockerfile

# Stage-specific validation (use custom script)
./scripts/hadolint_multistage.py Dockerfile

Common Multi-Stage Issues:

• Using same user across build and runtime stages
• Copying unnecessary build tools to production image
• Missing security hardening in final stage
• Secrets present in build stage propagating to runtime

When to use: Complex builds, security-hardened images, production containerization.

6. Pre-Commit Hook Integration

Prevent insecure Dockerfiles from being committed:

# Install pre-commit hook using bundled script
./scripts/install_precommit.sh

# Or manually create hook
cat << 'EOF' > .git/hooks/pre-commit
#!/bin/bash
for dockerfile in $(git diff --cached --name-only | grep -E 'Dockerfile'); do
  hadolint --failure-threshold warning "$dockerfile" || exit 1
done
EOF

chmod +x .git/hooks/pre-commit

When to use: Developer workstations, team onboarding, mandatory security controls.

Security Considerations

Sensitive Data Handling

• Secret Detection: Hadolint flags hardcoded secrets in ENV, ARG, LABEL instructions
• Build Secrets: Use Docker BuildKit secrets (RUN --mount=type=secret) instead of ARG for credentials
• Multi-Stage Security: Ensure secrets in build stages don't leak to final image
• Image Scanning: Hadolint validates Dockerfile - combine with image scanning (Trivy, Grype) for runtime security

Access Control

• CI/CD Permissions: Hadolint scans require read access to Dockerfile and build context
• Report Storage: Treat scan reports as internal documentation - may reveal security practices
• Trusted Registries: Configure trustedRegistries to enforce approved base image sources

Audit Logging

Log the following for compliance and security auditing:

• Scan execution timestamps and Dockerfile paths
• Rule violations by severity (error, warning, info)
• Suppressed rules and justifications
• Base image registry validation results
• Remediation actions and timeline

Compliance Requirements

• CIS Docker Benchmark 1.6: Hadolint rules map to CIS controls (see references/cis_mapping.md)
• 4.1: Create a user for the container (DL3002)
• 4.6: Add HEALTHCHECK instruction (DL3025)
• 4.7: Do not use update alone in Dockerfile (DL3009)
• 4.9: Use COPY instead of ADD (DL3020)
• OWASP Docker Security: Validates against OWASP container security best practices
• NIST SP 800-190: Application container security guidance

Bundled Resources

Scripts (scripts/)

• hadolint_scan.py - Comprehensive scanning with multiple Dockerfiles and output formats