CVE-2026-34810: Endian Firewall Stored XSS Vulnerability

CVE-2026-34810 Overview

Endian Firewall version 3.3.25 and prior versions contain a stored cross-site scripting (XSS) vulnerability in the remark parameter of /cgi-bin/vpnfw.cgi. An authenticated attacker can inject arbitrary JavaScript code that is stored on the server and executed when other users view the affected page. This vulnerability (CWE-79) allows attackers to hijack user sessions, steal credentials, perform actions on behalf of legitimate users, and potentially compromise the firewall administration interface.

Critical Impact
Authenticated attackers can inject persistent malicious scripts into the firewall administration interface, potentially compromising administrator accounts and gaining control over network security infrastructure.

Affected Products

Endian Firewall version 3.3.25
Endian Firewall versions prior to 3.3.25

Discovery Timeline

2026-04-02 - CVE-2026-34810 published to NVD
2026-04-02 - Last updated in NVD database

Technical Details for CVE-2026-34810

Vulnerability Analysis

This stored XSS vulnerability affects the VPN firewall configuration interface of Endian Firewall. The remark parameter in /cgi-bin/vpnfw.cgi fails to properly sanitize user-supplied input before storing and rendering it in the web interface. When an authenticated user submits malicious JavaScript code through this parameter, the script is stored in the application's backend and subsequently executed in the browsers of other users who access the affected page.

The attack requires low privileges—only authentication to the firewall interface—and leverages passive user interaction where victims simply need to view the compromised page. Because the malicious payload persists on the server, this represents a more dangerous variant of XSS compared to reflected attacks.

Root Cause

The root cause of this vulnerability is insufficient input validation and output encoding in the CGI script responsible for handling VPN firewall remarks. The application fails to sanitize special characters such as <, >, ", and ' in the remark parameter before storing the value and does not apply proper HTML entity encoding when rendering the stored content back to users.

Attack Vector

The attack is network-based, requiring an authenticated attacker to have access to the Endian Firewall web administration interface. The attacker submits a crafted request to /cgi-bin/vpnfw.cgi containing JavaScript code in the remark parameter. This malicious script is stored in the firewall configuration and executed whenever another user—particularly administrators—views the page containing the injected content.

Potential attack scenarios include:

Session hijacking by stealing authentication cookies
Credential theft via fake login forms
Privilege escalation by performing administrative actions on behalf of logged-in administrators
Defacement of the firewall management interface
Delivery of secondary payloads or redirection to malicious sites

Detection Methods for CVE-2026-34810

Indicators of Compromise

Unexpected JavaScript or HTML tags present in VPN firewall remark fields in configuration files or database
Unusual outbound connections from administrator workstations to unknown domains after accessing the firewall interface
Web server access logs showing encoded script tags (%3Cscript%3E) in POST requests to /cgi-bin/vpnfw.cgi
Reports from users about unexpected browser behavior when accessing the firewall administration panel

Detection Strategies

Monitor HTTP request logs for suspicious patterns in the remark parameter, particularly encoded or raw <script> tags and event handlers like onerror, onload, or onclick
Implement Content Security Policy (CSP) headers to detect and block inline script execution
Conduct regular audits of stored configuration data for unexpected HTML or JavaScript content
Deploy web application firewalls (WAF) with XSS detection rules to flag malicious input patterns

Monitoring Recommendations

Enable detailed logging for all CGI requests to /cgi-bin/vpnfw.cgi and review logs for anomalous input
Configure browser-based monitoring or endpoint detection to alert on suspicious script execution within the firewall management domain
Set up alerting for any changes to VPN firewall remarks or configuration that contain special characters associated with XSS payloads

How to Mitigate CVE-2026-34810

Immediate Actions Required

Restrict access to the Endian Firewall web administration interface to trusted IP addresses only
Review all existing VPN firewall remark entries for suspicious content and remove any injected scripts
Enforce strict role-based access control to limit the number of users who can modify VPN firewall configurations
Consider implementing an additional authentication layer such as VPN access before reaching the firewall management interface

Patch Information

No official vendor patch has been identified at this time. Organizations should monitor the Endian Community Help Section for security updates. Additional technical details are available in the VulnCheck Advisory: Endian Firewall XSS.

Workarounds

Deploy a web application firewall (WAF) in front of the Endian Firewall administration interface with rules to block XSS payloads
Implement strict Content Security Policy headers to prevent inline script execution
Use network segmentation to isolate the firewall management interface from general user access
Manually sanitize or disable the remark functionality if operationally feasible until an official patch is available

bash

# Example: Restrict access to firewall admin interface by IP (iptables)
iptables -A INPUT -p tcp --dport 443 -s 10.0.0.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 443 -j DROP