CVE-2019-25426

Executive Summary

CVE-2019-25426 is a reflected cross-site scripting (XSS) vulnerability in Comodo Dome Firewall version 2.7.0. The flaw exists in the dnsmasq endpoint, where user-supplied input to the TRANSPARENT_SOURCE_BYPASS or TRANSPARENT_DESTINATION_BYPASS parameters is not properly sanitized before being reflected back to the user. An attacker can craft a malicious POST request containing JavaScript payloads, which, when executed in a victim's browser, can lead to session hijacking, credential theft, or further compromise of the firewall's administrative interface. No patch has been publicly released, and the vendor has not confirmed a fix.

CVSS Breakdown & Threat Level

• CVSS Score: 4.7 (Medium)
• Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
• Attack Vector: Network
• Attack Complexity: Low
• Privileges Required: None
• User Interaction: Required
• Scope: Changed
• Confidentiality Impact: Low
• Integrity Impact: Low
• Availability Impact: None

The vulnerability is rated Medium due to the requirement for user interaction (clicking a crafted link) and the limited impact on confidentiality and integrity. However, in the context of a firewall management interface, successful exploitation could allow an attacker to perform administrative actions on behalf of an authenticated user.

Deep Dive & Mechanics

The vulnerability resides in the dnsmasq endpoint of the Comodo Dome Firewall web interface. When a POST request is sent to this endpoint with parameters TRANSPARENT_SOURCE_BYPASS or TRANSPARENT_DESTINATION_BYPASS, the server reflects the parameter values directly into the HTML response without proper encoding or sanitization. This allows an attacker to inject arbitrary HTML and JavaScript.

The attack flow:

1. An attacker crafts a malicious URL or form that sends a POST request to the vulnerable endpoint.
2. The victim, who is authenticated to the firewall's web interface, clicks the link or submits the form.
3. The injected script executes in the victim's browser, with access to the same origin as the firewall interface.
4. The script can steal session cookies, perform actions on behalf of the victim, or redirect to a phishing page.

Vulnerable vs. Patched Code

Vulnerable Code (Conceptual):

// dnsmasq.php
$source_bypass = $_POST['TRANSPARENT_SOURCE_BYPASS'];
echo "<input type='hidden' name='TRANSPARENT_SOURCE_BYPASS' value='$source_bypass'>";

Patched Code (Conceptual):

// dnsmasq.php
$source_bypass = htmlspecialchars($_POST['TRANSPARENT_SOURCE_BYPASS'], ENT_QUOTES, 'UTF-8');
echo "<input type='hidden' name='TRANSPARENT_SOURCE_BYPASS' value='$source_bypass'>";

The patched version uses htmlspecialchars() to encode special characters, preventing script injection.

Exploitation & Lab Setup

Red Team

Proof of Concept (PoC):

<html>
<body>
<form action="https://target-firewall/dnsmasq" method="POST">
  <input type="hidden" name="TRANSPARENT_SOURCE_BYPASS" value="<script>alert('XSS')</script>">
  <input type="submit" value="Click me">
</form>
</body>
</html>

Lab Setup:

1. Deploy a Comodo Dome Firewall 2.7.0 instance in a virtualized environment.
2. Access the web interface and authenticate as an admin.
3. Host the PoC HTML file on an attacker-controlled server.
4. Send the link to the victim (simulated admin) and trick them into clicking the submit button.
5. Observe the JavaScript execution (e.g., alert box).

Blue Team

Detection:

• Monitor for POST requests to /dnsmasq with suspicious parameters containing <script>, onerror, javascript:, or other XSS payloads.
• Use web application firewalls (WAF) to block requests with script tags in the TRANSPARENT_SOURCE_BYPASS or TRANSPARENT_DESTINATION_BYPASS parameters.

Mitigation:

• Apply input validation and output encoding on the server side.
• Implement Content Security Policy (CSP) headers to restrict script execution.
• Upgrade to a patched version if available; otherwise, consider isolating the management interface.

Indicators of Compromise (IOCs)

• Network:
  • POST requests to /dnsmasq with parameters containing <script>, %3Cscript%3E, or other HTML/JavaScript encodings.
  • Referer headers from untrusted domains.
• Host:
  • Unusual JavaScript execution in the browser console.
  • Unexpected redirects or pop-ups in the firewall management interface.
• Logs:
  • Web server logs showing repeated POST requests to /dnsmasq with non-standard parameter values.

Detection Engineering

YARA Rule

rule CVE_2019_25426_XSS {
    meta:
        description = "Detects reflected XSS attempts against Comodo Dome Firewall dnsmasq endpoint"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2019-25426"
    strings:
        $xss_payload = /TRANSPARENT_SOURCE_BYPASS|TRANSPARENT_DESTINATION_BYPASS/
        $script_tag = /<script[^>]*>/
    condition:
        $xss_payload and $script_tag
}

Sigma Rule

title: Reflected XSS Attempt on Comodo Dome Firewall
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects POST requests to /dnsmasq with XSS payloads in TRANSPARENT_SOURCE_BYPASS or TRANSPARENT_DESTINATION_BYPASS parameters
author: Valters IT
date: 2025-04-10
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'POST'
        cs-uri-query: '/dnsmasq'
    keywords:
        - 'TRANSPARENT_SOURCE_BYPASS'
        - 'TRANSPARENT_DESTINATION_BYPASS'
    xss_patterns:
        - '<script'
        - 'onerror='
        - 'javascript:'
    condition: selection and keywords and xss_patterns
falsepositives:
    - Legitimate use of similar parameter names in other applications
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2019-25426 - Reflected XSS in Comodo Dome Firewall dnsmasq"; flow:to_server; http.method; content:"POST"; http.uri; content:"/dnsmasq"; http.client_body; content:"TRANSPARENT_SOURCE_BYPASS"; nocase; http.client_body; content:"<script"; nocase; sid:1000001; rev:1;)

Remediation & Mitigation

1. Immediate Actions:

   • Restrict access to the firewall management interface to trusted IP addresses only.
   • Implement a Web Application Firewall (WAF) with rules to block XSS payloads.
   • Disable the dnsmasq endpoint if not required.

2. Long-term Fix:

   • Apply input validation and output encoding on the server side.
   • Upgrade to a patched version of Comodo Dome Firewall if one becomes available.
   • Conduct a security audit of the entire web interface for similar vulnerabilities.

3. Monitoring:

   • Enable detailed logging for all POST requests to the management interface.
   • Set up alerts for suspicious parameter values.

Valters IT