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156 lines
4.6 KiB
Markdown
156 lines
4.6 KiB
Markdown
---
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title: "Writeup - Road (THM)"
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date: 2022-04-08
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slug: "writeup-road-thm"
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type: "writeup-ctf"
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---
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This is a writeup for the [Road](https://tryhackme.com/room/road) machine from the TryHackMe site.
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## Enumeration
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First, let's start with a scan of our target with the following command:
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```bash
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nmap -sV -T4 -Pn 10.10.57.115
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```
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Two TCP ports are discovered:
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- 22/tcp : SSH port (OpenSSH 8.2p1)
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- 80/tcp : HTTP web server (Apache 2.4.41)
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## Exploit
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I start with an enumeration of the files of the website.
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I find a button on the basic site page that redirects to a login page. We have the possibility to create an account, I start by doing that.
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Once the account is created, I log in and see the following page:
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In the `edit profil` section you can't modify anything except the profile picture, but after looking closer, a message indicates that only the admins can do this action... Except that we learn an important information: the email of the admin!
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After some research on the site, I find another page. This page allows you to change your password. I make a password change and capture the request sent to the server with Burp.
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I realize that the email of the account is sent during the validation of the form, so I try to send the request but changing my email for the admin one. The server does not return any error, so I can connect to the admin account of the site!
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Now that I'm admin, I can upload a new profile picture!
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So I create a PHP reverse shell with the following template:
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[php-reverse-shell/php-reverse-shell.php at master · pentestmonkey/php-reverse-shellContribute to pentestmonkey/php-reverse-shell development by creating an account on GitHub.
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{{< github repo="pentestmonkey/php-reverse-shell" >}}
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I upload my `reverse.php` file thanks to the profile image change form. No error during the upload, I just have to find where the file has been put on the server..
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I look at the source code of the page to see if there would not be any information. I find the following comment:
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So I go to the following address:
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```bash
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10.10.57.115/v2/profileimages/reverse.php
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```
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I now have a reverse shell and can recover the first flag.
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```bash
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$ cat /home/webdeveloper/user.txt
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63191e4ece37523c9fe6bb62a5e64d45
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```
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## Privilege escalation
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I start by running [linPeas](https://linpeas.sh). In the result of the command I find that Mysql and MangoDB are running on the machine...
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I upgrade my shell with the following command:
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```bash
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python3 -c 'import pty; pty.spawn("/bin/bash")'
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```
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Then I try to connect to MySQL without success, so I test with MongoDB :
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I am now in Mongo, I list the databases with the following command:
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After a little exploration, I find in the `backup` database a table `user` :
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I can now connect via SSH to webdeveloper. I then check if this user has SUDO authorization:
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The `webdeveloper` user can execute the `sky_backup_utility` with root rights. But the most interesting thing is the tag: `env_keep+=LD_PRELOAD`.
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After some research I found this website:
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[Sudo (LD_PRELOAD) (Linux Privilege Escalation) – Touhid’s Blog](https://touhidshaikh.com/blog/2018/04/sudo-ld_preload-linux-privilege-escalation/)
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Overall, it explains that it is possible to execute code before the program and that with root execution rights. So I create a bash.c file with the following content :
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```C
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#include <stdio.h>
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#include <sys/types.h>
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#include <stdlib.h>
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void _init() {
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unsetenv("LD_PRELOAD");
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setgid(0);
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setuid(0);
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system("/bin/bash");
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}
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```
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bash.cThen I compile it with the following command:
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```bash
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gcc -fPIC -shared -o evil.so evil.c -nostartfiles
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```
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I can now run the program with sudo, without forgetting our code that will be executed at the beginning:
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```bash
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sudo LD_PRELOAD=/home/webdeveloper/bash.so sky_backup_utility
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```
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I now have a root shell so I can get the last flag.
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## Recommendations
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To patch this host I think it would be necessary to perform a number of actions:
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- Secured the password change page to prevent a user from changing the password of a user other than his own
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- Set up a verification of the upload files to the server to avoid sending PHP code or other
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- Do not store passwords in clear text in a database
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- Secure access to databases
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- Do not change SETUID bit of a program to avoid `LD_PRELOAD` exploit
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