Hardening Docker Containers For Production

Hardening Docker Containers for Production

Overview

Hardening Docker containers for production involves applying security best practices aligned with CIS Docker Benchmark v1.8.0 to minimize attack surface, prevent privilege escalation, and enforce least-privilege principles across Docker daemon, images, containers, and runtime configurations.

When to Use

• When deploying or configuring hardening docker containers for production capabilities in your environment
• When establishing security controls aligned to compliance requirements
• When building or improving security architecture for this domain
• When conducting security assessments that require this implementation

Common Misconfigurations & Verification

• Exposed daemon socket without mTLS: "hosts": ["tcp://0.0.0.0:2376"] with tlsverify missing/false is remote root. Verify: ss -tlnp | grep -E '2375|2376' and confirm docker info shows TLS enabled; 2375 (plaintext) should never listen.
• docker.sock mounted into a container: -v /var/run/docker.sock:/var/run/docker.sock hands the container host control. Verify: docker inspect <id> --format '{{.Mounts}}'.
• Capabilities re-added after drop: --cap-drop ALL --cap-add SYS_ADMIN (or NET_ADMIN) reintroduces escape primitives. Verify: docker inspect --format '{{.HostConfig.CapAdd}}'.
• USER overridden at runtime: a non-root Dockerfile USER 65534 is negated by docker run --user 0. Verify: docker inspect --format '{{.Config.User}}' and docker exec <id> id.
• Read-only rootfs with exec-able tmpfs: --read-only but a --tmpfs /tmp lacking noexec,nosuid still allows dropped-binary execution.
• no-new-privileges absent: confirm --security-opt no-new-privileges:true or daemon "no-new-privileges": true; check docker inspect --format '{{.HostConfig.SecurityOpt}}'.
• Verify the whole posture: run docker/docker-bench-security and treat any [WARN] on sections 2 (daemon), 4 (images), and 5 (runtime) as actionable; dockle and hadolint for the image/Dockerfile.

Prerequisites

• Docker Engine 24.0+ installed
• Docker Compose v2
• Linux host with kernel 5.10+
• Root or sudo access on Docker host
• docker-bench-security tool
• Hadolint for Dockerfile linting
• Dockle for image linting

Core Concepts

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