Docker Container Security

Docker / Container Security

A pragmatic baseline for Docker on a single VPS or a small cluster. Covers the Dockerfile, the run-time configuration, and the host-side gotchas — particularly the UFW-bypass that catches most people once.

When to invoke

• Installing Docker on a VPS that has UFW (read the UFW section first — Docker bypasses UFW by default)
• Writing a new Dockerfile or docker-compose.yml for production
• Pushing an image to a public registry
• Periodic audit of running containers
• After a base-image CVE that affects your stack

The UFW bypass — read this first

Docker manipulates iptables directly. By default, ports published with -p are exposed to the world, even if UFW says they should not be. ufw status will mislead you.

Two options:

Option A — use ufw-docker (community-maintained, robust):

# Install ufw-docker
sudo wget -O /usr/local/bin/ufw-docker \
  https://github.com/chaifeng/ufw-docker/raw/master/ufw-docker
sudo chmod +x /usr/local/bin/ufw-docker
sudo ufw-docker install
sudo systemctl restart ufw

# Then allow per-container:
sudo ufw-docker allow web 80/tcp

Option B — bind to localhost when you front with a reverse proxy:

# docker-compose.yml — bind to 127.0.0.1, not 0.0.0.0
services:
  app:
    ports:
      - "127.0.0.1:9000:9000"   # host nginx proxies to this

Verify:

sudo ss -tlnp | grep docker     # should not show 0.0.0.0:<port> for internal services

Dockerfile baseline

Every production Dockerfile should:

1. Run as a non-root user
2. Pin the base image to a specific digest or version tag (not latest)
3. Drop build tools from the final image
4. Not contain secrets baked in

# Pin to specific Node version; consider digest pinning for stricter supply chain
FROM node:20.11.1-bookworm-slim AS deps
WORKDIR /app
COPY package.json package-lock.json ./
RUN npm ci --omit=dev

FROM node:20.11.1-bookworm-slim
WORKDIR /app

# Create non-root user
RUN groupadd -r app && useradd -r -g app -d /app -s /sbin/nologin app

COPY --from=deps /app/node_modules ./node_modules
COPY --chown=app:app . .

USER app
EXPOSE 3000

# Use exec form so the process becomes PID 1 and receives signals
CMD ["node", "server.js"]

Notes:

• -slim is much smaller than the default tag. Distroless (gcr.io/distroless/nodejs20-debian12) is smaller and shell-less — harder to live in if an attacker gets RCE
• Multi-stage to leave compilers / dev deps out of the runtime image
• --chown on COPY so files are not owned by root inside the image
• USER last so all later instructions run as that user

Run-time hardening

For each container, prefer the most restrictive run-time options the app actually tolerates.

docker run -d \
  --read-only \
  --tmpfs /tmp:rw,noexec,nosuid,size=64m \
  --cap-drop=ALL \
  --cap-add=NET_BIND_SERVICE \              # only if the app needs to bind <1024
  --security-opt no-new-privileges:true \
  --pids-limit 200 \
  --memory 512m \
  --cpus 1 \
  -p 127.0.0.1:3000:3000 \
  --name myapp myapp:1.2.3

In docker-compose.yml:

services:
  app:
    image: myapp:1.2.3
    read_only: true
    tmpfs:
      - /tmp:size=64m,noexec,nosuid
    cap_drop: [ALL]
    cap_add: [NET_BIND_SERVICE]
    security_opt:
      - no-new-privileges:true
    pids_limit: 200
    mem_limit: 512m
    cpus: 1.0
    user: "1000:1000"
    restart: unless-stopped

What each flag does:

• --read-only — root filesystem cannot be modified. Writable dirs are explicit tmpfs mounts or volumes.
• --cap-drop=ALL + selective --cap-add — drop all Linux capabilities, add only what the app needs. Most apps need none.
• --security-opt no-new-privileges — process cannot gain privileges via setuid binaries.
• --pids-limit — caps fork-bomb DoS.
• --memory / --cpus — prevents one runaway container from killing the host.

Secrets — never bake them in

Anything in ENV or ARG ends up in image layers. Anyone who can pull the image can extract them.

# Bad
docker build --build-arg STRIPE_KEY=sk_live_... .

# Good — at runtime, from env
docker run -e STRIPE_KEY="$STRIPE_KEY" myapp

# Better — Docker secrets (Swarm) or mounted files
docker run -v /run/secrets/stripe:/run/secrets/stripe:ro myapp
# app reads /run/secrets/stripe

# Best — a real secret manager (Vault, cloud KMS) with short-lived tokens

To inspect a built image for accidental secrets:

docker history --no-trunc myapp:1.2.3
docker save myapp:1.2.3 -o myapp.tar
tar -xf myapp.tar
# search the layer tarballs for .env / known token prefixes

Image scanning

Run a scanner on every image before deploy. Trivy is the most common choice and is fast enough for CI.

trivy image --severity HIGH,CRITICAL --exit-code 1 myapp:1.2.3

In GitHub Actions:

- name: Build image
  run: docker build -t myapp:${{ github.sha }} .
- name: Scan with Trivy
  uses: aquasecurity/trivy-action@<pinned-sha>
  with:
    image-ref: 'myapp:${{ github.sha }}'
    severity: 'HIGH,CRITICAL'
    exit-code: '1'

Scan on a schedule too — vulnerabilities are discovered after images are built. A weekly re-scan catches newly-known CVEs in already-deployed images.

Network segmentation

Default Docker bridge network puts all containers on one flat L2 segment. They can reach each other freely.

# docker-compose.yml — isolate concerns
networks:
  frontend:
  backend:
    internal: true                # no external connectivity
  database:
    internal: true

services:
  web:
    networks: [frontend, backend]
  api:
    networks: [backend, database]
  db:
    networks: [database]

internal: true networks have no NAT to the outside — only useful for intra-cluster communication. The DB cannot connect out to the internet, which limits exfil if compromised.

Periodic audit

# What is running with --privileged or as root?
docker ps --format 'table {{.Names}}\t{{.Image}}' | tail -n +2 | while read name img; do
  user=$(docker inspect --format '{{.Config.User}}' "$name")
  priv=$(docker inspect --format '{{.HostConfig.Privileged}}' "$name")
  echo "$name  image=$img  user='${user:-<root>}'  privileged=$priv"
done

# Containers with publicly-bound ports
docker ps --format '{{.Names}}\t{{.Ports}}' | grep -E '0\.0\.0\.0|:::' || echo "OK - none"

# Images older than 90 days
docker images --format '{{.Repository}}:{{.Tag}} {{.CreatedSince}}' | grep -E '(months|year)'

# Stopped containers + dangling images — clean up unless you need forensic state
docker container prune -f --filter 'until=720h'   # 30 days
docker image prune -f

Compose-file checklist

Before deploying any docker-compose.yml to production, walk this:

• No latest tags; versions are pinned (and ideally digest-pinned for critical images)
• Every service has user: set to a non-root UID:GID
• read_only: true wherever the app tolerates it (most do)
• cap_drop: [ALL] + minimal cap_add
• security_opt: [no-new-privileges:true]
• mem_limit and pids_limit set
• Ports bound to 127.0.0.1 unless intentionally public
• No secrets in environment: — use env_file: (gitignored), Docker secrets, or a manager
• Internal networks (internal: true) used to keep DB/cache off the public bridge
• restart: unless-stopped rather than always (so manual stops stick)
• Volumes are named, not anonymous (named volumes survive docker compose down, anonymous do not)

What this skill will not do

• Help bypass container isolation on systems you do not own
• Recommend --privileged, --cap-add=SYS_ADMIN, or running as root unless there is a documented, narrow need
• Replace a full Kubernetes security review (different stack, different controls)