CVE-2026-34813: Endian Firewall Stored XSS Vulnerability

CVE-2026-34813 Overview

A stored cross-site scripting (XSS) vulnerability has been identified in Endian Firewall version 3.3.25 and prior versions. The vulnerability exists in the user parameter of /cgi-bin/proxyuser.cgi, allowing authenticated attackers to inject arbitrary JavaScript code that is stored on the server and executed when other users view the affected page.

Critical Impact

An authenticated attacker can inject malicious JavaScript that persists on the firewall management interface, potentially compromising administrative sessions, stealing credentials, or performing unauthorized actions on behalf of other users.

Affected Products

• Endian Firewall version 3.3.25
• Endian Firewall versions prior to 3.3.25

Discovery Timeline

• 2026-04-02 - CVE-2026-34813 published to NVD
• 2026-04-02 - Last updated in NVD database

Technical Details for CVE-2026-34813

Vulnerability Analysis

This stored XSS vulnerability (CWE-79) occurs due to improper neutralization of user-supplied input before it is stored and subsequently rendered in web pages. The vulnerability requires network access and low-privilege authentication to exploit, with user interaction required when the victim views the compromised page. The attack targets the subsequent system context, potentially affecting other users' sessions when they access the vulnerable page.

The stored nature of this XSS vulnerability makes it particularly concerning in a firewall management context, as malicious scripts persist across sessions and can affect multiple administrators who access the compromised page.

Root Cause

The root cause of this vulnerability is insufficient input validation and output encoding in the proxyuser.cgi script. When user input is submitted via the user parameter, the application fails to properly sanitize the input before storing it in the database and fails to encode it when rendering the content back to users. This allows JavaScript code embedded in the parameter to be executed in the browser context of any user who views the affected page.

Attack Vector

The attack is network-based and requires the attacker to have authenticated access to the Endian Firewall management interface. The attacker submits a crafted request to /cgi-bin/proxyuser.cgi with malicious JavaScript embedded in the user parameter. This payload is stored on the server and executed whenever another user (typically an administrator) views the page containing the injected content.

The stored XSS attack flow involves the attacker authenticating to the firewall interface with valid credentials, injecting JavaScript payload through the vulnerable user parameter, the server storing the malicious content without sanitization, and subsequently any user viewing the affected page having the script execute in their browser context with their session privileges.

Detection Methods for CVE-2026-34813

Indicators of Compromise

• Unusual JavaScript content in database fields related to proxy user configurations
• HTTP requests to /cgi-bin/proxyuser.cgi containing encoded script tags or JavaScript event handlers in the user parameter
• Web server logs showing suspicious payloads such as <script>, onerror=, onload=, or other XSS vectors

Detection Strategies

• Implement web application firewall (WAF) rules to detect and block XSS payloads in requests to /cgi-bin/proxyuser.cgi
• Monitor authentication logs for unusual patterns of access followed by configuration changes
• Review stored user data in the proxy configuration for unexpected HTML or JavaScript content
• Deploy Content Security Policy (CSP) headers to mitigate the impact of successful XSS attacks

Monitoring Recommendations

• Enable detailed logging for all CGI script interactions on the Endian Firewall
• Configure alerts for requests containing common XSS payload patterns targeting the management interface
• Regularly audit stored user input fields for signs of injection attempts
• Monitor for unexpected administrative actions that may indicate session hijacking

How to Mitigate CVE-2026-34813

Immediate Actions Required

• Restrict access to the Endian Firewall management interface to trusted networks only
• Review existing proxy user configurations for signs of injected malicious content
• Implement network segmentation to limit exposure of the firewall management interface
• Consider deploying a reverse proxy with XSS filtering capabilities in front of the management interface

Patch Information

At the time of publication, no vendor patch has been confirmed. Organizations should monitor the Endian Community Support portal for security updates. Additional technical details are available in the VulnCheck Advisory on Endian Firewall.

Workarounds

• Limit management interface access to specific trusted IP addresses using firewall rules
• Implement additional authentication layers such as VPN requirements before accessing the management interface
• Disable or restrict access to /cgi-bin/proxyuser.cgi if proxy user management functionality is not required
• Deploy browser-based XSS protection extensions for administrators who must access the interface

# Example: Restrict management interface access using iptables
# Allow only trusted admin network to access web interface
iptables -A INPUT -p tcp --dport 443 -s 192.168.1.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 443 -j DROP