CVE-2021-46902

Executive Summary

CVE-2021-46902 is a path validation vulnerability in the LTOS-Web-Interface component of Meinberg LANTIME-Firmware. The flaw allows an authenticated administrator to read or delete arbitrary files on the system, bypassing intended access controls. This is a classic path traversal (CWE-22) issue that can lead to unauthorized file access or deletion, potentially compromising system integrity and confidentiality. The vulnerability affects firmware versions before 6.24.029 (for the 6.x branch) and before 7.04.008 (for the 7.x branch). No public proof-of-concept or exploit code is available at this time.

CVSS Breakdown & Threat Level

CVSS Score: 0.0 (No official CVSS vector assigned by NVD) Threat Level: High (based on impact and administrative privileges required)

Rationale: While the CVSS score is listed as 0.0, the vulnerability allows an admin to read or delete arbitrary files. This is a significant security risk as it can lead to:

• Confidentiality Impact: High (unauthorized file reading)
• Integrity Impact: High (unauthorized file deletion)
• Availability Impact: High (deletion of critical system files)

EPSS Score: 0.0% (Very low probability of exploitation in the wild)

Deep Dive & Mechanics

The vulnerability resides in the LTOS-Web-Interface, which is the web-based management interface for Meinberg LANTIME devices. The core issue is improper validation of file paths when an administrator performs file read or delete operations through the web interface.

Root Cause: The application fails to properly sanitize user-supplied input used to construct file paths. Specifically, it does not adequately check for path traversal sequences (e.g., ../) or absolute paths. This allows an authenticated admin to specify paths outside the intended directory, such as /etc/passwd or /etc/shadow.

Attack Vector:

1. An attacker gains administrative access to the LANTIME web interface (legitimate credentials or via another vulnerability).
2. The attacker navigates to a file management function (e.g., "Read File" or "Delete File").
3. The attacker inputs a malicious path like ../../etc/passwd or /etc/shadow.
4. The application processes the request without proper validation, allowing the attacker to read or delete the targeted file.

Impact:

• Read: An admin can read sensitive system files (configuration files, password hashes, SSH keys, etc.).
• Delete: An admin can delete critical system files, potentially causing a denial of service or system instability.

Vulnerable vs. Patched Code

Vulnerable Code (Conceptual):

// Vulnerable LTOS-Web-Interface code
$file = $_POST['file_path'];
$content = file_get_contents($file); // No path validation
echo $content;

Patched Code (Conceptual):

// Patched LTOS-Web-Interface code
$file = $_POST['file_path'];
$base_dir = '/var/www/ltos/data/';
$real_path = realpath($base_dir . $file);
if (strpos($real_path, $base_dir) !== 0) {
    die('Invalid path');
}
$content = file_get_contents($real_path);
echo $content;

The patched code uses realpath() to resolve the absolute path and then checks that it starts with the allowed base directory. This prevents path traversal attacks.

Exploitation & Lab Setup

Red Team

Prerequisites:

• Administrative access to the LANTIME web interface.
• Network access to the device.

Exploitation Steps:

1. Log in to the LTOS-Web-Interface as an admin.
2. Identify the file management endpoint (e.g., /cgi-bin/file_manager.cgi).
3. Craft a POST request with a malicious file path:

   POST /cgi-bin/file_manager.cgi HTTP/1.1
   Host: target-lantime
   Content-Type: application/x-www-form-urlencoded
   Cookie: admin_session=valid_session
   
   action=read&file=../../etc/passwd
   

4. The server will return the contents of /etc/passwd.

Alternative Payloads:

• Read SSH private keys: ../../etc/ssh/ssh_host_rsa_key
• Delete configuration: action=delete&file=../../etc/config/network.conf

Blue Team

Detection:

• Monitor web server logs for unusual file paths containing ../ or absolute paths.
• Alert on file read/delete operations targeting sensitive system files (e.g., /etc/passwd, /etc/shadow, /etc/ssh/).
• Implement file integrity monitoring (FIM) on critical system files.

Prevention:

• Apply the vendor patch immediately.
• If patching is not possible, restrict administrative access to trusted IPs only.
• Implement a web application firewall (WAF) rule to block path traversal patterns.

Indicators of Compromise (IOCs)

Network IOCs:

• HTTP requests containing ../ or absolute paths in file management endpoints.
• Unusual file read/delete operations from the LANTIME web interface.

File System IOCs:

• Unexpected modifications or deletions of system files (e.g., /etc/passwd, /etc/shadow, /etc/ssh/).
• Presence of backup files created by an attacker (e.g., /etc/passwd.bak).

Log IOCs:

• Web server logs showing requests like:

  GET /cgi-bin/file_manager.cgi?action=read&file=../../etc/passwd
  

• System logs showing file access violations.

Detection Engineering

YARA Rule

rule CVE_2021_46902_LANTIME_Path_Traversal {
    meta:
        description = "Detects path traversal attempts in LTOS-Web-Interface logs"
        author = "Valters IT"
        date = "2024-01-01"
        reference = "CVE-2021-46902"
    strings:
        $path_traversal = /\.\.\/|\.\.\\|%2e%2e%2f|%2e%2e%5c/
        $file_operation = /action=(read|delete)/
    condition:
        $path_traversal and $file_operation
}

Sigma Rule

title: CVE-2021-46902 - LANTIME Path Traversal Attempt
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects path traversal attempts targeting Meinberg LANTIME devices
author: Valters IT
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'GET' or 'POST'
        cs-uri-query|contains: 
            - '../'
            - '..\\'
            - '%2e%2e%2f'
            - '%2e%2e%5c'
        cs-uri-query|contains|all:
            - 'action=read'
            - 'action=delete'
    condition: selection
falsepositives:
    - Legitimate file management operations within allowed directories
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (
    msg:"CVE-2021-46902 - LANTIME Path Traversal Attempt";
    flow:to_server,established;
    content:"action=read"; http_uri;
    content:"action=delete"; http_uri;
    pcre:"/(\.\.\/|\.\.\\|%2e%2e%2f|%2e%2e%5c)/i";
    classtype:attempted-admin;
    sid:1000001;
    rev:1;
)

Remediation & Mitigation

Immediate Actions:

1. Patch: Upgrade to LANTIME-Firmware 6.24.029 (MBGID-9343) or 7.04.008 (MBGID-6303) as applicable.
2. Workaround: If patching is not immediately possible, restrict administrative access to the web interface to trusted IP addresses only.
3. Monitor: Enable detailed logging on the web server and monitor for path traversal attempts.

Long-Term Measures:

• Implement a web application firewall (WAF) with rules to block path traversal patterns.
• Conduct regular security audits of the LTOS-Web-Interface code.
• Apply the principle of least privilege to file system access.
• Use file integrity monitoring (FIM) on critical system files.

Vendor Contact: Meinberg (https://www.meinberg.com)

Valters IT