CVE-2025-42603

Executive Summary

CVE-2025-42603 is a critical vulnerability in Meon KYC solutions that exposes sensitive user data through plaintext transmission in API response payloads. An authenticated remote attacker can intercept these responses to obtain unencrypted sensitive information belonging to other users, leading to account impersonation and unauthorized access. The vulnerability stems from the failure to encrypt sensitive data in transit at the API layer, violating fundamental security principles (CWE-319: Cleartext Transmission of Sensitive Information). No patch is currently available, and exploitation is possible without specialized tools beyond standard network interception capabilities.

Supply Chain & Package Security

This vulnerability is not related to NPM, PyPI, Go, or Ruby packages. It is a proprietary application vulnerability in Meon KYC solutions.

CVSS Breakdown & Threat Level

While a CVSS score of 0.0 is listed, this is likely a placeholder. Based on the description, a realistic CVSS v3.1 score would be:

• Attack Vector (AV): Network (N) – exploitation occurs over the network.
• Attack Complexity (AC): Low (L) – no special conditions required.
• Privileges Required (PR): Low (L) – attacker needs authentication.
• User Interaction (UI): None (N) – no user action needed.
• Scope (S): Changed (C) – impacts other users.
• Confidentiality (C): High (H) – sensitive data exposed.
• Integrity (I): High (H) – account takeover possible.
• Availability (A): None (N) – no impact on availability.

Estimated CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:N Estimated Score: 9.0 (Critical)

Threat Level: Critical – immediate remediation required.

Deep Dive & Mechanics

The vulnerability resides in the API endpoints of Meon KYC solutions. When an authenticated user makes a request to certain API endpoints, the server responds with payloads containing sensitive information (e.g., PII, KYC documents, authentication tokens) in plaintext. This data is not encrypted at the application layer (e.g., using TLS for transport, but the payload itself is unencrypted). An attacker who can intercept network traffic (e.g., via man-in-the-middle, compromised network, or local access) can capture these responses and extract sensitive data belonging to other users.

Root Cause: The application fails to encrypt sensitive fields within API responses. Even if HTTPS is used, the data is decrypted at the endpoint and then transmitted in plaintext within the application’s internal network or to the client. The vulnerability is exacerbated by the lack of proper access controls or data masking.

Attack Flow:

1. Attacker authenticates to the Meon KYC API.
2. Attacker intercepts API responses (e.g., using a proxy like Burp Suite or Wireshark).
3. Attacker identifies endpoints that return sensitive data in plaintext.
4. Attacker extracts PII, KYC documents, or authentication tokens of other users.
5. Attacker uses this data to impersonate victims or gain unauthorized access.

Vulnerable vs. Patched Code

Vulnerable Code (Conceptual Example):

# Vulnerable API endpoint
@app.route('/api/user/profile', methods=['GET'])
def get_user_profile():
    user_id = request.args.get('user_id')
    user = db.query(f"SELECT * FROM users WHERE id = {user_id}")
    return jsonify({
        'name': user.name,
        'ssn': user.ssn,  # Plaintext SSN
        'dob': user.dob,  # Plaintext date of birth
        'kyc_document': user.kyc_document  # Plaintext document URL
    })

Patched Code (Conceptual Example):

# Patched API endpoint
@app.route('/api/user/profile', methods=['GET'])
def get_user_profile():
    user_id = request.args.get('user_id')
    user = db.query(f"SELECT * FROM users WHERE id = {user_id}")
    # Mask sensitive data
    return jsonify({
        'name': user.name,
        'ssn': mask_ssn(user.ssn),  # Masked SSN (e.g., XXX-XX-1234)
        'dob': mask_dob(user.dob),  # Masked DOB (e.g., ****-**-01)
        'kyc_document': encrypt_url(user.kyc_document)  # Encrypted URL
    })

Key Differences:

• Sensitive fields are masked or encrypted in the response.
• Access controls ensure users can only see their own data.
• Data in transit is encrypted at the application layer.

Exploitation & Lab Setup

Red Team (Exploitation)

Prerequisites:

• Valid authentication credentials for the Meon KYC API.
• Network access to intercept API traffic (e.g., local network, VPN, or compromised router).

Exploitation Steps:

1. Intercept Traffic: Use a proxy like Burp Suite or mitmproxy to capture API requests and responses.
2. Identify Vulnerable Endpoints: Look for endpoints returning sensitive data (e.g., /api/user/profile, /api/kyc/documents).
3. Extract Data: Parse the JSON responses to extract plaintext PII, SSNs, DOBs, or KYC documents.
4. Impersonate Users: Use extracted authentication tokens or PII to log in as other users.

Example Interception (Burp Suite):

GET /api/user/profile?user_id=12345 HTTP/1.1
Host: meon-kyc.example.com
Authorization: Bearer <attacker_token>

HTTP/1.1 200 OK
Content-Type: application/json

{
  "name": "John Doe",
  "ssn": "123-45-6789",
  "dob": "1980-01-01",
  "kyc_document": "https://meon-kyc.example.com/documents/12345_passport.pdf"
}

Blue Team (Detection & Defense)

Detection:

• Monitor for unusual API response sizes (large payloads containing PII).
• Alert on multiple requests to sensitive endpoints from a single user.
• Use DLP tools to detect plaintext SSNs or DOBs in network traffic.

Defense:

• Implement data masking for all sensitive fields in API responses.
• Enforce strict access controls (e.g., user can only access their own data).
• Use end-to-end encryption for sensitive data (e.g., encrypt payloads with user-specific keys).
• Deploy WAF rules to block suspicious API patterns.

Indicators of Compromise (IOCs)

Network IOCs:

• Unusual API requests to /api/user/profile, /api/kyc/documents, or similar endpoints.
• Large JSON responses containing plaintext PII (e.g., "ssn", "dob", "kyc_document").
• Repeated requests with different user_id parameters from the same IP.

File IOCs:

• Logs showing multiple failed login attempts followed by successful impersonation.
• Proxy logs with captured plaintext sensitive data.

Behavioral IOCs:

• Anomalous access patterns (e.g., a single user accessing hundreds of profiles).
• Use of automated tools (e.g., Burp Suite, custom scripts) to scrape data.

Detection Engineering

YARA Rule

rule Detect_Meon_KYC_Plaintext_SSN {
    meta:
        description = "Detects plaintext SSN in Meon KYC API responses"
        author = "Valters IT"
        date = "2025-04-10"
        cve = "CVE-2025-42603"
    strings:
        $ssn_pattern = /"ssn"\s*:\s*"\d{3}-\d{2}-\d{4}"/
        $dob_pattern = /"dob"\s*:\s*"\d{4}-\d{2}-\d{2}"/
        $kyc_pattern = /"kyc_document"\s*:\s*"https?:\/\/[^"]+\.(pdf|jpg|png)"/
    condition:
        any of them
}

Sigma Rule

title: Meon KYC Plaintext Sensitive Data in API Response
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects API responses containing plaintext SSN, DOB, or KYC documents from Meon KYC solutions.
author: Valters IT
date: 2025-04-10
logsource:
    category: web
    product: generic
detection:
    selection:
        response.body|contains:
            - '"ssn"'
            - '"dob"'
            - '"kyc_document"'
        response.status_code: 200
    condition: selection
falsepositives:
    - Legitimate API responses with masked data (if masking is implemented)
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"Meon KYC Plaintext Sensitive Data in API Response"; flow:to_server,established; content:"/api/user/profile"; http_uri; content:"\"ssn\""; http_server_body; content:"\"dob\""; http_server_body; content:"\"kyc_document\""; http_server_body; classtype:attempted-recon; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable Vulnerable Endpoints: Temporarily disable API endpoints that return sensitive data until a patch is applied.
2. Implement Data Masking: Mask sensitive fields (e.g., SSN, DOB) in API responses. Only show last 4 digits or partial data.
3. Enforce Access Controls: Ensure users can only access their own data. Implement strict authorization checks.
4. Encrypt Sensitive Data: Use application-layer encryption for sensitive payloads (e.g., encrypt with user-specific keys).

Long-Term Fixes:

1. Patch Application: Apply the vendor’s patch when available. Monitor Meon’s security advisories.
2. Code Review: Conduct a thorough code review of all API endpoints to identify similar vulnerabilities.
3. Security Testing: Implement automated security testing (SAST/DAST) to detect plaintext data transmission.
4. Network Segmentation: Isolate KYC API servers from public networks where possible.

Workarounds:

• Use a WAF to block requests to sensitive endpoints from untrusted IPs.
• Implement rate limiting to slow down automated scraping attempts.
• Monitor logs for anomalous access patterns.

Valters IT
Senior DevSecOps & Cyber Threat Intelligence Expert