CVE-2026-34807: Endian Firewall Stored XSS Vulnerability

CVE-2026-34807 Overview

A stored cross-site scripting (XSS) vulnerability exists in Endian Firewall version 3.3.25 and prior versions. The vulnerability is present in the remark parameter of /cgi-bin/incoming.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 type of persistent XSS attack can lead to session hijacking, credential theft, and unauthorized actions performed on behalf of legitimate users.

Critical Impact

Authenticated attackers can inject malicious JavaScript that persists and executes in the context of other users' sessions, potentially compromising administrator accounts and firewall configurations.

Affected Products

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

Discovery Timeline

• 2026-04-02 - CVE-2026-34807 published to NVD
• 2026-04-02 - Last updated in NVD database

Technical Details for CVE-2026-34807

Vulnerability Analysis

This vulnerability is classified as CWE-79 (Improper Neutralization of Input During Web Page Generation), commonly known as Cross-Site Scripting. The stored XSS variant is particularly dangerous because the malicious payload persists in the application's database or filesystem, affecting all users who subsequently view the compromised page.

The vulnerable endpoint /cgi-bin/incoming.cgi fails to properly sanitize user-supplied input in the remark parameter before storing and later rendering it in web pages. This allows an authenticated attacker to inject JavaScript code that will execute in the browsers of other users, including administrators, when they access the affected page.

Root Cause

The root cause of this vulnerability is insufficient input validation and output encoding in the Endian Firewall web interface. The remark parameter accepts user input that is stored without sanitization and later rendered directly into HTML pages without proper encoding. This allows script tags and other HTML elements to be injected and interpreted by browsers as executable code rather than text content.

Attack Vector

The attack requires network access and authentication to the Endian Firewall web interface. Once authenticated, an attacker can submit a crafted request to /cgi-bin/incoming.cgi with malicious JavaScript embedded in the remark parameter. The payload is stored server-side and executes whenever another authenticated user views the page containing the remark content.

The attacker could leverage this vulnerability to steal session cookies, capture keystrokes, redirect users to phishing pages, or perform actions within the firewall administration interface using the victim's privileges. If an administrator's session is compromised, the attacker could potentially modify firewall rules, create backdoor access, or extract sensitive configuration data.

For detailed technical information about this vulnerability, refer to the VulnCheck Advisory on Endian Firewall.

Detection Methods for CVE-2026-34807

Indicators of Compromise

• Unusual JavaScript code or HTML tags present in remark fields within the Endian Firewall database or logs
• Unexpected outbound connections from administrator workstations after accessing the firewall web interface
• Web server access logs showing requests to /cgi-bin/incoming.cgi with encoded script tags or suspicious payloads in the remark parameter
• User reports of unexpected browser behavior or redirects when using the firewall interface

Detection Strategies

• Implement web application firewall (WAF) rules to detect and block XSS payloads in HTTP requests targeting /cgi-bin/incoming.cgi
• Monitor and audit the remark field contents in the firewall database for suspicious patterns such as <script>, javascript:, or event handler attributes
• Deploy endpoint detection solutions to identify malicious JavaScript execution in browser contexts when accessing firewall management interfaces
• Review HTTP access logs for anomalous POST requests to the vulnerable CGI endpoint with encoded special characters

Monitoring Recommendations

• Enable detailed logging for all CGI script access on the Endian Firewall appliance
• Configure alerting for multiple failed or unusual authentication attempts followed by successful logins
• Implement session monitoring to detect cookie exfiltration attempts or session replay attacks
• Conduct regular content audits of user-supplied data fields to identify injected payloads

How to Mitigate CVE-2026-34807

Immediate Actions Required

• Restrict access to the Endian Firewall web interface to trusted networks and IP addresses only
• Review and audit all existing remark entries for malicious content and sanitize any suspicious data
• Implement additional authentication controls such as multi-factor authentication for firewall administration
• Consider placing a reverse proxy with XSS filtering capabilities in front of the firewall management interface

Patch Information

No official patch information is currently available for this vulnerability. Users should monitor the Endian Community Help Section for updates and security advisories from the vendor. Additionally, consult the VulnCheck Advisory on Endian Firewall for the latest information regarding patches or workarounds.

Workarounds

• Implement network segmentation to limit access to the firewall management interface to a dedicated management VLAN
• Deploy a web application firewall (WAF) with XSS detection rules in front of the Endian Firewall web interface
• Disable or restrict access to the /cgi-bin/incoming.cgi endpoint if the functionality is not required
• Use browser security extensions that block or warn about suspicious script execution when administering the firewall

# Example: Restrict access to management interface via iptables
# Run on a perimeter device or the firewall itself if accessible via CLI
iptables -A INPUT -p tcp --dport 443 -s 10.0.0.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 443 -j DROP