Hardening Docker Containers For Production

CIS Docker Benchmark Sections

1. Host Configuration - Audit Docker daemon files, restrict access to /var/run/docker.sock
2. Docker Daemon Configuration - Enable TLS, restrict inter-container communication, configure logging
3. Docker Daemon Configuration Files - Set ownership and permissions on daemon.json
4. Container Images and Build File - Use trusted base images, scan for vulnerabilities, multi-stage builds
5. Container Runtime - Drop capabilities, read-only rootfs, restrict syscalls
6. Docker Security Operations - Monitor, audit, and rotate credentials

Key Hardening Principles

• Least Privilege: Run containers as non-root, drop all capabilities except required
• Immutability: Use read-only root filesystem, tmpfs for writable directories
• Minimalism: Use distroless or Alpine base images, multi-stage builds
• Isolation: Apply seccomp profiles, AppArmor/SELinux, namespace restrictions
• Auditability: Enable content trust, log all container activity

Workflow

Step 1: Harden the Dockerfile

# Use specific digest for reproducibility
FROM python:3.12-slim@sha256:abc123... AS builder

WORKDIR /app
COPY requirements.txt .
RUN pip install --no-cache-dir --user -r requirements.txt

# Production stage - minimal image
FROM gcr.io/distroless/python3-debian12

# Copy only necessary artifacts
COPY --from=builder /root/.local /root/.local
COPY --from=builder /app /app

WORKDIR /app

# Create non-root user
USER 65534:65534

# Set read-only filesystem expectation
LABEL org.opencontainers.image.source="https://github.com/org/app"

ENTRYPOINT ["python", "app.py"]

Step 2: Harden Docker Daemon Configuration

{
  "icc": false,
  "log-driver": "json-file",
  "log-opts": {
    "max-size": "10m",
    "max-file": "3"
  },
  "live-restore": true,
  "userland-proxy": false,
  "no-new-privileges": true,
  "default-ulimits": {
    "nofile": {
      "Name": "nofile",
      "Hard": 64000,
      "Soft": 64000
    },
    "nproc": {
      "Name": "nproc",
      "Hard": 1024,
      "Soft": 1024
    }
  },
  "seccomp-profile": "/etc/docker/seccomp-default.json",
  "tls": true,
  "tlscacert": "/etc/docker/tls/ca.pem",
  "tlscert": "/etc/docker/tls/server-cert.pem",
  "tlskey": "/etc/docker/tls/server-key.pem",
  "tlsverify": true
}

Step 3: Harden Container Runtime

docker run -d \
  --name production-app \
  --read-only \
  --tmpfs /tmp:rw,noexec,nosuid,size=100m \
  --tmpfs /var/run:rw,noexec,nosuid,size=10m \
  --cap-drop ALL \
  --cap-add NET_BIND_SERVICE \
  --security-opt no-new-privileges:true \
  --security-opt seccomp=/etc/docker/seccomp-default.json \
  --security-opt apparmor=docker-default \
  --pids-limit 100 \
  --memory 512m \
  --memory-swap 512m \
  --cpus 1.0 \
  --user 65534:65534 \
  --network custom-bridge \
  --restart on-failure:3 \
  --health-cmd "curl -f http://localhost:8080/health || exit 1" \
  --health-interval 30s \
  --health-timeout 10s \
  --health-retries 3 \
  myapp:latest

Step 4: Enable Docker Content Trust

export DOCKER_CONTENT_TRUST=1
export DOCKER_CONTENT_TRUST_SERVER=https://notary.example.com

# Sign and push image
docker trust sign myregistry.com/myapp:v1.0.0

# Verify image signature before pull
docker trust inspect --pretty myregistry.com/myapp:v1.0.0

Step 5: Configure Host-Level Auditing

# Add audit rules for Docker files and directories
cat >> /etc/audit/rules.d/docker.rules << 'EOF'
-w /usr/bin/docker -k docker
-w /var/lib/docker -k docker
-w /etc/docker -k docker
-w /lib/systemd/system/docker.service -k docker
-w /lib/systemd/system/docker.socket -k docker
-w /etc/default/docker -k docker
-w /etc/docker/daemon.json -k docker
-w /usr/bin/containerd -k docker
-w /usr/bin/runc -k docker
EOF

systemctl restart auditd

Validation Commands

# Run Docker Bench Security
docker run --rm --net host --pid host \
  --userns host --cap-add audit_control \
  -e DOCKER_CONTENT_TRUST=$DOCKER_CONTENT_TRUST \
  -v /etc:/etc:ro \
  -v /usr/bin/containerd:/usr/bin/containerd:ro \
  -v /usr/bin/runc:/usr/bin/runc:ro \
  -v /usr/lib/systemd:/usr/lib/systemd:ro \
  -v /var/lib:/var/lib:ro \
  -v /var/run/docker.sock:/var/run/docker.sock:ro \
  docker/docker-bench-security

# Lint Dockerfile
hadolint Dockerfile

# Lint built image
dockle myapp:latest

# Verify no containers running as root
docker ps -q | xargs docker inspect --format '{{.Id}}: User={{.Config.User}}'

Key Security Controls

References

• CIS Docker Benchmark
• Docker Security Best Practices
• Docker Bench Security Tool
• Hadolint - Dockerfile Linter