Docker Security Guide

🚨 CRITICAL GUIDELINES

Windows File Path Requirements

MANDATORY: Always Use Backslashes on Windows for File Paths

When using Edit or Write tools on Windows, you MUST use backslashes (\) in file paths, NOT forward slashes (/).

Examples:

• ❌ WRONG: D:/repos/project/file.tsx
• ✅ CORRECT: D:\repos\project\file.tsx

This applies to:

• Edit tool file_path parameter
• Write tool file_path parameter
• All file operations on Windows systems

Documentation Guidelines

NEVER create new documentation files unless explicitly requested by the user.

• Priority: Update existing README.md files rather than creating new documentation
• Repository cleanliness: Keep repository root clean - only README.md unless user requests otherwise
• Style: Documentation should be concise, direct, and professional - avoid AI-generated tone
• User preference: Only create additional .md files when user specifically asks for documentation

---

Docker Security Guide

This skill provides comprehensive security guidelines for Docker across all platforms, covering threats, mitigations, and compliance requirements.

Security Principles

Defense in Depth

Apply security at multiple layers: 1. Image security: Minimal, scanned, signed images 2. Build security: Secure build process, no secrets in layers 3. Runtime security: Restricted capabilities, resource limits 4. Network security: Isolation, least privilege 5. Host security: Hardened host OS, updated Docker daemon 6. Orchestration security: Secure configuration, RBAC 7. Monitoring: Detection, logging, alerting

Least Privilege

Grant only the minimum permissions necessary:

• Non-root users
• Dropped capabilities
• Read-only filesystems
• Minimal network exposure
• Restricted syscalls (seccomp)
• Limited resources

Image Security

Base Image Selection

Threat: Vulnerable or malicious base images

Mitigation:

# Use official images only
FROM node:20.11.0-alpine3.19  # Official, specific version

# NOT
FROM randomuser/node  # Unverified source
FROM node:latest      # Unpredictable, can break

Verification:

# Verify image source
docker image inspect node:20-alpine | grep -A 5 "Author"

# Enable Docker Content Trust (image signing)
export DOCKER_CONTENT_TRUST=1
docker pull node:20-alpine

Minimal Images

Threat: Larger attack surface, more vulnerabilities

Mitigation:

# Prefer minimal distributions
FROM alpine:3.19           # ~7MB
FROM gcr.io/distroless/static  # ~2MB
FROM scratch               # 0MB (for static binaries)

# vs
FROM ubuntu:22.04          # ~77MB with more packages

Benefits:

• Fewer packages = fewer vulnerabilities
• Smaller attack surface
• Faster downloads and starts
• Less disk space

Micro-Distros for Security-Critical Applications (2025)

Wolfi/Chainguard Images:

• Zero-CVE goal, SBOM included by default
• Nightly security patches, signed with provenance
• Available for: Node, Python, Go, Java, .NET, etc.

Usage:

# Development stage (includes build tools)
FROM cgr.dev/chainguard/node:latest-dev AS builder
WORKDIR /app
COPY package*.json ./
RUN npm ci --only=production

# Production stage (minimal, zero-CVE goal)
FROM cgr.dev/chainguard/node:latest
WORKDIR /app
COPY --from=builder /app/node_modules ./node_modules
COPY . .
USER node
ENTRYPOINT ["node", "server.js"]

When to use: Security-critical apps, compliance requirements (SOC2, HIPAA, PCI-DSS), zero-trust environments, supply chain security emphasis.

See docker-best-practices skill for full image comparison table.

Vulnerability Scanning

Tools:

• Docker Scout (built-in)
• Trivy
• Grype
• Snyk
• Clair

Process:

# Scan with Docker Scout
docker scout cves IMAGE_NAME
docker scout recommendations IMAGE_NAME

# Scan with Trivy
trivy image IMAGE_NAME
trivy image --severity HIGH,CRITICAL IMAGE_NAME

# Scan Dockerfile
trivy config Dockerfile

# Scan for secrets
trivy fs --scanners secret .

CI/CD Integration:

# GitHub Actions example
- name: Scan image
  run: |
    docker scout cves my-image:${{ github.sha }}
    trivy image --exit-code 1 --severity CRITICAL my-image:${{ github.sha }}

Multi-Stage Builds for Security

Threat: Build tools and secrets in final image

Mitigation:

# Build stage with build tools
FROM golang:1.21 AS builder
WORKDIR /app
COPY . .
RUN go build -o app

# Final stage - minimal, no build tools
FROM gcr.io/distroless/base-debian11
COPY --from=builder /app/app /
USER nonroot:nonroot
ENTRYPOINT ["/app"]

Benefits:

• No compiler/build tools in production image
• Secrets used in build don't persist
• Smaller, more secure final image

Build-Time & Runtime Security

Complete recipes for build secrets (--mount=type=secret, BuildKit), multi-stage hardening, capability drops (--cap-drop=ALL), seccomp / AppArmor profiles, read-only root, user namespaces, resource limits, and rootless runtime live in references/build-runtime-security.md. Core principles:

• Build secrets: use --mount=type=secret and --mount=type=ssh; never ARG/ENV for sensitive values.
• Runtime: drop all capabilities and add back what is needed; run read-only root; non-root user; resource limits; pinned image digests.
• Profiles: apply seccomp, AppArmor, and SELinux; combine with user namespaces.

See references/build-runtime-security.md for all commands and recipes.

Network Security

Network Isolation

Threat: Lateral movement between containers

Mitigation:

networks:
  frontend:
    driver: bridge
  backend:
    driver: bridge
    internal: true  # No external access

services:
  web:
    networks:
      - frontend

  api:
    networks:
      - frontend
      - backend

  database:
    networks:
      - backend  # Isolated from frontend

Port Exposure

Threat: Unnecessary network exposure

Mitigation:

# Bind to localhost only
docker run -p 127.0.0.1:8080:8080 my-image

# NOT (binds to all interfaces)
docker run -p 8080:8080 my-image

In Compose:

services:
  app:
    ports:
      - "127.0.0.1:8080:8080"  # Localhost only

Inter-Container Communication

# Disable default inter-container communication
# /etc/docker/daemon.json
{
  "icc": false
}

Then explicitly allow via networks:

services:
  app1:
    networks:
      - app-network
  app2:
    networks:
      - app-network  # Can communicate with app1

networks:
  app-network:
    driver: bridge

Secrets Management

Docker Secrets (Swarm Mode)

# Create secret
echo "mypassword" | docker secret create db_password -

# Use in service
docker service create \
  --name my-service \
  --secret db_password \
  my-image

# Access in container at /run/secrets/db_password

In stack file:

version: '3.8'

services:
  app:
    image: my-image
    secrets:
      - db_password

secrets:
  db_password:
    external: true

Secrets Best Practices

1. Never in environment variables (visible in docker inspect) 2. Never in images (in layer history) 3. Never in version control (Git history) 4. Mount as files with restricted permissions 5. Use secret management systems (Vault, AWS Secrets Manager, etc.) 6. Rotate regularly

Alternative: Mounted secrets:

docker run -v /secure/secrets:/run/secrets:ro my-image

Compliance & Benchmarking

CIS Docker Benchmark

Automated checking:

# Clone docker-bench-security
git clone https://github.com/docker/docker-bench-security.git
cd docker-bench-security
sudo sh docker-bench-security.sh

# Or run as container
docker run --rm --net host --pid host --userns host \
  --cap-add audit_control \
  -v /var/lib:/var/lib:ro \
  -v /var/run/docker.sock:/var/run/docker.sock:ro \
  -v /usr/lib/systemd:/usr/lib/systemd:ro \
  -v /etc:/etc:ro \
  docker/docker-bench-security

Key CIS Recommendations

1. Host Configuration

• Keep Docker up to date
• Restrict network traffic between containers
• Set logging level to 'info'
• Enable Docker Content Trust

2. Docker Daemon

• Use TLS for Docker daemon socket
• Don't expose daemon on TCP without TLS
• Enable user namespace support

3. Docker Files

• Verify Docker files ownership and permissions
• Audit Docker files and directories

4. Container Images

• Create user for container
• Use trusted base images
• Don't install unnecessary packages

5. Container Runtime

• Run containers with limited privileges
• Set resource limits
• Don't share host network namespace

Monitoring & Detection

Logging

services:
  app:
    logging:
      driver: "json-file"
      options:
        max-size: "10m"
        max-file: "3"
        labels: "service,env"
        env: "ENV,VERSION"

Centralized logging:

services:
  app:
    logging:
      driver: "syslog"
      options:
        syslog-address: "tcp://log-server:514"
        tag: "{{.Name}}/{{.ID}}"

Runtime Monitoring

Tools:

• Falco: Runtime security monitoring
• Sysdig: Container visibility
• Prometheus + cAdvisor: Metrics
• Docker events: Real-time events

Monitor for:

• Unexpected processes
• File modifications
• Network connections
• Resource spikes
• Failed authentication
• Privilege escalation attempts

# Monitor Docker events
docker events --filter 'type=container' --filter 'event=start'

# Watch specific container
docker events --filter "container=my-container"

# Runtime security with Falco
docker run --rm -it \
  --privileged \
  -v /var/run/docker.sock:/host/var/run/docker.sock \
  -v /dev:/host/dev \
  -v /proc:/host/proc:ro \
  falcosecurity/falco

Platform-Specific Security

Linux

User namespace remapping:

// /etc/docker/daemon.json
{
  "userns-remap": "default"
}

Benefits: Root in container → unprivileged on host

SELinux:

# Enable SELinux for Docker
setenforce 1

# Run with SELinux labels
docker run --security-opt label=type:svirt_sandbox_file_t my-image

# Volumes with SELinux
docker run -v /host/path:/container/path:z my-image

AppArmor:

# Check AppArmor status
aa-status

# Run with AppArmor profile
docker run --security-opt apparmor=docker-default my-image

Windows

Hyper-V isolation:

# More isolated than process isolation
docker run --isolation=hyperv my-image

Windows Defender:

• Ensure real-time protection enabled
• Configure exclusions carefully
• Scan images regularly

macOS

Docker Desktop security:

• Keep Docker Desktop updated
• Enable "Use gRPC FUSE for file sharing"
• Limit file sharing to necessary paths
• Review resource allocation

Security Checklist

Image:

• [ ] Based on official, minimal image
• [ ] Specific version tag (not latest)
• [ ] Scanned for vulnerabilities
• [ ] No secrets in layers
• [ ] Runs as non-root user
• [ ] Signed (Content Trust)

Build:

• [ ] .dockerignore configured
• [ ] Multi-stage build (if applicable)
• [ ] Build secrets handled properly
• [ ] Build from trusted sources only

Runtime:

• [ ] Non-root user
• [ ] Capabilities dropped
• [ ] Read-only filesystem (where possible)
• [ ] Security options set
• [ ] Resource limits configured
• [ ] Isolated network
• [ ] Minimal port exposure
• [ ] Secrets mounted securely

Operations:

• [ ] CIS benchmark compliance
• [ ] Logging configured
• [ ] Monitoring in place
• [ ] Regular vulnerability scans
• [ ] Incident response plan
• [ ] Regular updates
• [ ] Audit logs enabled

Common Security Mistakes

❌ NEVER:

• Run as root
• Use --privileged
• Mount Docker socket (/var/run/docker.sock)
• Hardcode secrets
• Use latest tag
• Skip vulnerability scanning
• Expose unnecessary ports
• Disable security features
• Ignore security updates
• Trust unverified images

✅ ALWAYS:

• Run as non-root
• Drop capabilities
• Scan for vulnerabilities
• Use secrets management
• Tag with specific versions
• Enable security options
• Apply least privilege
• Keep systems updated
• Monitor runtime behavior
• Use official images

This security guide represents current best practices. Security threats evolve constantly—always check the latest Docker security documentation and CVE databases.

Build-Time & Runtime Security (Detailed Reference)

Complete recipes for Docker build-time secrets, multi-stage hardening, BuildKit isolation, capability drops, seccomp / AppArmor profiles, read-only root, user namespaces, resource limits, and rootless runtime. SKILL.md keeps the principle headlines; this reference has the full commands and examples.

Build-Time Security

Secrets Management

NEVER:

# BAD - Secret in layer history
ENV API_KEY=abc123
RUN git clone https://user:password@github.com/repo.git
COPY .env /app/.env

DO:

# Use BuildKit secrets
# syntax=docker/dockerfile:1

FROM alpine
RUN --mount=type=secret,id=github_token \
    git clone https://$(cat /run/secrets/github_token)@github.com/repo.git

# Build with secret (not in image)
docker build --secret id=github_token,src=./token.txt .

BuildKit Frontend Security (2025)

Threat: Malicious or compromised BuildKit frontends can execute arbitrary code during build

🚨 2025 CRITICAL WARNING: BuildKit supports custom frontends (parsers) via # syntax= directive. Untrusted frontends have FULL BUILD-TIME code execution and can:

• Steal secrets from build context
• Modify build outputs
• Exfiltrate data
• Compromise the build environment

Risk Example:

# 🔴 DANGER - Untrusted frontend (code execution risk!)
# syntax=docker/dockerfile:1@sha256:abc123...untrusted

FROM alpine
RUN echo "This frontend could do anything during build"

Mitigation:

1. Only use official Docker frontends:

# ✅ Safe - Official Docker frontend
# syntax=docker/dockerfile:1

# ✅ Safe - Specific version
# syntax=docker/dockerfile:1.5

# ✅ Safe - Pinned with digest (verify from docker.com)
# syntax=docker/dockerfile:1@sha256:ac85f380a63b13dfcefa89046420e1781752bab202122f8f50032edf31be0021

2. Verify frontend sources:

• Use ONLY docker/dockerfile:* frontends
• Pin to specific versions with SHA256 digest
• Verify digests from official Docker documentation
• Never use third-party frontends without thorough vetting

3. Audit all Dockerfiles for unsafe syntax directives:

# Check all Dockerfiles for potentially malicious syntax directives
grep -r "^# syntax=" . --include="Dockerfile*"

# Verify all frontends are official Docker images
grep -r "^# syntax=" . --include="Dockerfile*" | grep -v "docker/dockerfile"

4. BuildKit security configuration (defense in depth):

# Restrict frontend sources in BuildKit config
# /etc/buildkit/buildkitd.toml
[frontend."dockerfile.v0"]
  # Only allow official Docker frontends
  allowedImages = ["docker.io/docker/dockerfile:*"]

Supply Chain Protection:

• Treat custom frontends as HIGH RISK code execution vectors
• Review ALL # syntax= directives in Dockerfiles before builds
• Use content trust for frontend images
• Monitor for frontend vulnerabilities
• Include frontend verification in CI/CD security gates

SBOM (Software Bill of Materials) Generation (2025)

Critical 2025 Requirement: Document origin and history of all components for supply chain transparency and compliance.

Why SBOM is Mandatory:

• Supply chain security visibility
• Vulnerability tracking and response
• Compliance requirements (Executive Order 14028, etc.)
• License compliance
• Incident response readiness

Generate SBOM with Docker Scout:

# Generate SBOM for image
docker scout sbom IMAGE_NAME

# Export SBOM in different formats
docker scout sbom --format spdx IMAGE_NAME > sbom.spdx.json
docker scout sbom --format cyclonedx IMAGE_NAME > sbom.cyclonedx.json

# Include SBOM attestation during build
# ⚠️ WARNING: BuildKit attestations are NOT cryptographically signed!
docker buildx build \
  --sbom=true \
  --provenance=true \
  --tag my-image:latest \
  .

# View SBOM attestations (unsigned metadata only)
docker buildx imagetools inspect my-image:latest --format "{{ json .SBOM }}"

🚨 CRITICAL SECURITY LIMITATION: BuildKit attestations (--sbom=true, --provenance=true) are NOT cryptographically signed. This means:

• Anyone with push access can create tampered attestations
• SBOMs can be incomplete or falsified
• Provenance data cannot be trusted without external verification
• For production: Use external signing tools (cosign, Notary) and Syft for SBOM generation

Generate SBOM with Syft:

# Install Syft
curl -sSfL https://raw.githubusercontent.com/anchore/syft/main/install.sh | sh

# Generate SBOM from image
syft my-image:latest

# Generate in specific format
syft my-image:latest -o spdx-json > sbom.spdx.json
syft my-image:latest -o cyclonedx-json > sbom.cyclonedx.json

# Generate from Dockerfile
syft dir:. -o spdx-json > sbom.spdx.json

SBOM in CI/CD Pipeline:

# GitHub Actions example
name: Build with SBOM

jobs:
  build:
    steps:
      - name: Build image with SBOM
        run: |
          docker buildx build \
            --sbom=true \
            --provenance=true \
            --tag my-image:${{ github.sha }} \
            --push \
            .

      - name: Generate SBOM with Syft
        run: |
          syft my-image:${{ github.sha }} -o spdx-json > sbom.json

      - name: Upload SBOM artifact
        uses: actions/upload-artifact@v3
        with:
          name: sbom
          path: sbom.json

      - name: Scan SBOM for vulnerabilities
        run: |
          grype sbom:sbom.json --fail-on high

SBOM Best Practices:

1. Generate for every image:

• Production images: mandatory
• Development images: recommended
• Base images: critical

2. Store SBOMs with provenance:

• Version control alongside Dockerfile
• Artifact registry with image
• Dedicated SBOM repository

3. Automate SBOM generation:

• Integrate into CI/CD pipeline
• Generate on every build
• Fail builds if SBOM generation fails

4. Use SBOM for vulnerability management:

# Scan SBOM instead of image (faster)
grype sbom:sbom.json
trivy sbom sbom.json

# Compare SBOMs between versions
diff <(syft old-image:1.0 -o json) <(syft new-image:2.0 -o json)

5. SBOM formats:

• SPDX: Industry standard, ISO/IEC 5962:2021
• CycloneDX: OWASP standard, security-focused
• Choose based on compliance requirements

Chainguard Images with Built-in SBOM:

# Chainguard images include SBOM attestation by default
docker buildx imagetools inspect cgr.dev/chainguard/node:latest

# Extract SBOM
cosign download sbom cgr.dev/chainguard/node:latest > chainguard-node-sbom.json

Or use multi-stage and don't include secrets:

FROM node AS builder
ARG NPM_TOKEN
RUN echo "//registry.npmjs.org/:_authToken=${NPM_TOKEN}" > .npmrc && \
    npm install && \
    rm .npmrc  # Still in layer history!

# Better - secret only in build stage
FROM node AS dependencies
RUN --mount=type=secret,id=npmrc,target=/root/.npmrc \
    npm install

FROM node AS runtime
COPY --from=dependencies /app/node_modules ./node_modules
# No .npmrc in final image

Secure Build Context

Threat: Sensitive files included in build context

Mitigation: Create comprehensive .dockerignore:

# Secrets
.env
.env.local
*.key
*.pem
credentials.json
secrets/

# Version control
.git
.gitignore

# Cloud credentials
.aws/
.gcloud/

# Private data
database.sql
backups/

# SSH keys
.ssh/
id_rsa
id_rsa.pub

# Sensitive logs
*.log
logs/

Image Signing

Enable Docker Content Trust:

# Enable image signing
export DOCKER_CONTENT_TRUST=1

# Set up keys
docker trust key generate my-key
docker trust signer add --key my-key.pub my-name my-image

# Push signed image
docker push my-image:tag

# Pull only signed images
docker pull my-image:tag  # Fails if not signed

Runtime Security

User Privileges

Threat: Container escape via root

Mitigation:

# Create and use non-root user
FROM node:20-alpine
RUN addgroup -g 1001 appuser && \
    adduser -S appuser -u 1001 -G appuser
USER appuser
WORKDIR /home/appuser/app
COPY --chown=appuser:appuser . .
CMD ["node", "server.js"]

Verification:

# Check user in running container
docker exec container-name whoami  # Should not be root
docker exec container-name id       # Check UID/GID

Capabilities

Threat: Excessive kernel capabilities

Default Docker capabilities:

• CHOWN, DAC_OVERRIDE, FOWNER, FSETID
• KILL, SETGID, SETUID, SETPCAP
• NET_BIND_SERVICE, NET_RAW
• SYS_CHROOT, MKNOD, AUDIT_WRITE, SETFCAP

Mitigation:

# Drop all, add only needed
docker run \
  --cap-drop=ALL \
  --cap-add=NET_BIND_SERVICE \
  my-image

In docker-compose.yml:

services:
  app:
    cap_drop:
      - ALL
    cap_add:
      - NET_BIND_SERVICE

Common needed capabilities:

• NET_BIND_SERVICE: Bind to ports < 1024
• NET_ADMIN: Network configuration
• SYS_TIME: Set system time

Read-Only Filesystem

Threat: Container modification, malware persistence

Mitigation:

docker run \
  --read-only \
  --tmpfs /tmp:noexec,nosuid,size=64M \
  --tmpfs /var/run:noexec,nosuid,size=64M \
  my-image

In Compose:

services:
  app:
    read_only: true
    tmpfs:
      - /tmp:noexec,nosuid,size=64M
      - /var/run:noexec,nosuid,size=64M

Security Options

no-new-privileges:

docker run --security-opt="no-new-privileges:true" my-image

Prevents privilege escalation via setuid/setgid binaries.

AppArmor (Linux):

docker run --security-opt="apparmor=docker-default" my-image

SELinux (Linux):

docker run --security-opt="label=type:container_runtime_t" my-image

Seccomp (syscall filtering):

# Use default profile
docker run --security-opt="seccomp=default" my-image

# Or custom profile
docker run --security-opt="seccomp=./seccomp-profile.json" my-image

Resource Limits

Threat: DoS via resource exhaustion

Mitigation:

docker run \
  --memory="512m" \
  --memory-swap="512m" \  # Disable swap
  --cpus="1.0" \
  --pids-limit=100 \
  --ulimit nofile=1024:1024 \
  my-image

In Compose:

services:
  app:
    deploy:
      resources:
        limits:
          cpus: '1.0'
          memory: 512M
          pids: 100
        reservations:
          cpus: '0.5'
          memory: 256M
    ulimits:
      nofile:
        soft: 1024
        hard: 1024

Comprehensive Secure Run Command

docker run \
  --name secure-app \
  --detach \
  --restart unless-stopped \
  --user 1000:1000 \
  --cap-drop=ALL \
  --cap-add=NET_BIND_SERVICE \
  --read-only \
  --tmpfs /tmp:noexec,nosuid,size=64M \
  --security-opt="no-new-privileges:true" \
  --security-opt="seccomp=default" \
  --memory="512m" \
  --cpus="1.0" \
  --pids-limit=100 \
  --network=isolated-network \
  --publish 127.0.0.1:8080:8080 \
  --volume secure-data:/data:ro \
  --health-cmd="curl -f http://localhost/health || exit 1" \
  --health-interval=30s \
  my-secure-image:1.2.3