CVE-2025-42601

Executive Summary

CVE-2025-42601 is a security advisory affecting Meon KYC (Know Your Customer) solutions. The vulnerability stems from insufficient server-side validation of the Captcha mechanism in certain API endpoints. A remote, unauthenticated attacker can intercept a legitimate API request, remove the Captcha parameter, and bypass the Captcha verification entirely. This allows automated scripts or bots to submit requests without solving the Captcha, potentially leading to account enumeration, credential stuffing, or other automated abuse. The vulnerability has a CVSS score of 0.0 (likely unassigned or placeholder), but the impact is rated as medium due to the bypass of a security control. No public exploit code or patch details are available at this time.

Supply Chain & Package Security

This vulnerability is not related to an open-source package (NPM, PyPI, Go, Ruby). It affects a proprietary KYC solution from Meon. No supply chain implications are identified.

CVSS Breakdown & Threat Level

• CVSS Score: 0.0 (Unassigned/Placeholder)
• Vector String: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
• Attack Vector (AV): Network – Exploitable remotely.
• Attack Complexity (AC): Low – No special conditions required.
• Privileges Required (PR): None – No authentication needed.
• User Interaction (UI): None – No user action required.
• Scope (S): Unchanged – Exploit does not affect other components.
• Confidentiality (C): None – No data disclosure.
• Integrity (I): Low – Captcha bypass allows automated submissions.
• Availability (A): None – No denial of service.

Threat Level: Medium. The vulnerability enables bypass of a security control, facilitating automated attacks such as credential stuffing, account enumeration, or spam submissions.

Deep Dive & Mechanics

The vulnerability exists in Meon KYC API endpoints that implement Captcha verification. The server-side code fails to enforce the presence or validity of the Captcha token. When a client sends a request with a valid Captcha token, the server processes it. However, if an attacker intercepts the request (e.g., via a proxy) and removes the Captcha parameter, the server accepts the request without verification.

Root Cause: The server does not check whether the Captcha parameter exists or is valid before processing the request. This is a classic case of missing server-side validation.

Attack Flow:

1. Attacker identifies an API endpoint that requires Captcha (e.g., /api/register, /api/login).
2. Attacker sends a legitimate request with a Captcha token.
3. Using a proxy (e.g., Burp Suite), the attacker intercepts the request and removes the captcha parameter.
4. The modified request is forwarded to the server.
5. The server processes the request without verifying the Captcha, allowing the attacker to bypass the security control.

Impact: Automated scripts can flood the API with requests, enabling credential stuffing, account enumeration, or spam. This undermines the KYC process.

Vulnerable vs. Patched Code

Vulnerable Code (Conceptual):

# Vulnerable endpoint
@app.route('/api/register', methods=['POST'])
def register():
    # Captcha verification is optional
    captcha_token = request.form.get('captcha')
    if captcha_token:
        if not verify_captcha(captcha_token):
            return jsonify({'error': 'Invalid captcha'}), 400
    # Process registration without captcha
    # ...

Patched Code (Conceptual):

# Patched endpoint
@app.route('/api/register', methods=['POST'])
def register():
    # Captcha verification is mandatory
    captcha_token = request.form.get('captcha')
    if not captcha_token:
        return jsonify({'error': 'Captcha required'}), 400
    if not verify_captcha(captcha_token):
        return jsonify({'error': 'Invalid captcha'}), 400
    # Process registration
    # ...

The patched code ensures that the Captcha parameter is always required and validated before processing the request.

Exploitation & Lab Setup

Red Team (Attacker)

Prerequisites:

• Burp Suite or similar proxy tool.
• Target Meon KYC API endpoint.

Steps:

1. Intercept a legitimate request to the vulnerable API endpoint (e.g., POST /api/register).
2. Remove the captcha parameter from the request body.
3. Forward the modified request.
4. Observe that the server accepts the request without Captcha verification.

Example using cURL:

# Legitimate request with captcha
curl -X POST https://target.com/api/register \
  -d "username=test&password=pass&captcha=valid_token"

# Bypass request without captcha
curl -X POST https://target.com/api/register \
  -d "username=test&password=pass"

Blue Team (Defender)

Detection:

• Monitor for requests to API endpoints that lack the captcha parameter.
• Alert on high frequency of requests from a single IP without Captcha tokens.
• Implement rate limiting and anomaly detection.

Mitigation:

• Enforce server-side Captcha validation for all sensitive endpoints.
• Use a Web Application Firewall (WAF) to block requests missing required parameters.
• Log and alert on missing Captcha parameters.

Indicators of Compromise (IOCs)

• Network:
  • HTTP requests to /api/register, /api/login, or similar endpoints without a captcha parameter.
  • High volume of POST requests from a single IP address.
• Logs:
  • Server logs showing requests with missing captcha field.
  • Failed Captcha verification attempts (if any).
• Behavioral:
  • Automated submissions (e.g., credential stuffing) bypassing Captcha.

Detection Engineering

YARA Rule

rule Detect_Captcha_Bypass_Request {
    meta:
        description = "Detects HTTP requests missing captcha parameter in Meon KYC API"
        author = "Valters IT"
        date = "2025-04-01"
        reference = "CVE-2025-42601"
    strings:
        $api_path = /\/api\/(register|login|verify|submit)/ nocase
        $missing_captcha = "captcha" not in request.body
    condition:
        $api_path and $missing_captcha
}

Sigma Rule

title: Meon KYC Captcha Bypass Attempt
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects HTTP requests to Meon KYC API endpoints missing the captcha parameter
author: Valters IT
date: 2025-04-01
logsource:
    category: webserver
    product: apache
    service: access
detection:
    selection:
        cs-method: 'POST'
        cs-uri-stem|startswith: '/api/'
        cs-uri-query|contains: 'captcha'
    condition: not selection
falsepositives:
    - Legitimate requests from clients that have already solved captcha (unlikely)
level: medium

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (
    msg:"Meon KYC Captcha Bypass Attempt";
    flow:to_server,established;
    content:"POST"; http_method;
    content:"/api/"; http_uri;
    pcre:"/captcha/";
    sid:1000001;
    rev:1;
    reference:cve,2025-42601;
    classtype:attempted-recon;
    priority:2;
)

Remediation & Mitigation

1. Immediate:
   • Implement mandatory server-side Captcha validation for all API endpoints.
   • Ensure the Captcha parameter is required and verified before processing.
2. Short-term:
   • Deploy a WAF rule to block requests missing the captcha parameter.
   • Enable rate limiting on sensitive endpoints.
3. Long-term:
   • Conduct a security audit of all API endpoints for similar validation issues.
   • Implement a security development lifecycle (SDL) with mandatory input validation.
4. Detection:
   • Monitor logs for missing Captcha parameters.
   • Set up alerts for high-frequency requests without Captcha.

Valters IT
Securing Digital Frontiers