CVE-2026-34818: Endian Firewall Stored XSS Vulnerability

CVE-2026-34818 Overview

CVE-2026-34818 is a stored cross-site scripting (XSS) vulnerability affecting Endian Firewall version 3.3.25 and prior versions. The vulnerability exists in the remark parameter of the /manage/dnsmasq/localdomains/ endpoint. An authenticated attacker can inject arbitrary JavaScript code that is stored on the server and executed whenever other users view the affected page, potentially leading to session hijacking, credential theft, or unauthorized actions performed on behalf of legitimate users.

Critical Impact

Authenticated attackers can inject persistent malicious JavaScript into the firewall management interface, compromising any administrator who views the affected page and potentially gaining full control of the firewall appliance.

Affected Products

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

Discovery Timeline

• 2026-04-02 - CVE CVE-2026-34818 published to NVD
• 2026-04-02 - Last updated in NVD database

Technical Details for CVE-2026-34818

Vulnerability Analysis

This stored XSS vulnerability (CWE-79) occurs due to improper neutralization of user-supplied input before it is rendered in HTML output. The /manage/dnsmasq/localdomains/ endpoint accepts a remark parameter that is stored in the application's backend and subsequently displayed to users without proper encoding or sanitization. When administrators or other authenticated users navigate to pages displaying these remarks, the injected JavaScript executes within their browser session in the context of the firewall management interface.

The vulnerability requires authentication for exploitation, meaning an attacker must first obtain valid credentials to the Endian Firewall management interface. However, once exploited, the stored nature of the XSS ensures that malicious payloads persist and can affect multiple victims over time without further attacker interaction.

Root Cause

The root cause of CVE-2026-34818 is insufficient input validation and output encoding in the DNSmasq local domains management functionality. The application fails to properly sanitize the remark parameter before storing it in the database and does not apply appropriate HTML entity encoding when rendering the stored value back to users. This allows attackers to inject script tags or event handlers that execute when the content is displayed.

Attack Vector

The attack vector for this vulnerability is network-based and requires the attacker to be authenticated to the Endian Firewall management interface. The attack flow typically involves:

1. An authenticated attacker accesses the local domains management page at /manage/dnsmasq/localdomains/
2. The attacker crafts a malicious payload containing JavaScript code and submits it via the remark parameter
3. The malicious payload is stored in the application's database without proper sanitization
4. When other administrators or users view the affected page, the stored JavaScript executes in their browser
5. The attacker can leverage the executed script to steal session cookies, perform actions as the victim, or exfiltrate sensitive configuration data

For detailed technical information about this vulnerability, see the VulnCheck Endian Advisory.

Detection Methods for CVE-2026-34818

Indicators of Compromise

• Unusual JavaScript code or HTML tags present in the remark field entries within DNSmasq local domain configurations
• Unexpected outbound network connections from administrator browsers when accessing the management interface
• Anomalous session activity or unauthorized configuration changes following administrator access to /manage/dnsmasq/localdomains/
• Presence of script injection patterns such as <script>, onerror=, or javascript: in application logs or database entries

Detection Strategies

• Implement web application firewall (WAF) rules to detect and block XSS payloads in HTTP requests to /manage/dnsmasq/localdomains/
• Enable Content Security Policy (CSP) headers to prevent inline script execution and report violations
• Deploy endpoint detection and response (EDR) solutions to monitor for suspicious browser behavior when accessing the firewall management interface
• Review application logs for requests containing encoded or obfuscated JavaScript payloads targeting the remark parameter

Monitoring Recommendations

• Monitor HTTP request logs for suspicious patterns in POST requests to /manage/dnsmasq/localdomains/
• Set up alerts for CSP violation reports indicating attempted script injection
• Review audit logs for unusual changes to DNSmasq local domain configurations
• Track administrator session activity for signs of session hijacking or unauthorized actions

How to Mitigate CVE-2026-34818

Immediate Actions Required

• Audit all existing entries in the DNSmasq local domains configuration for malicious JavaScript or HTML content
• Restrict access to the Endian Firewall management interface to trusted networks and IP addresses only
• Implement additional network-level controls such as VPN requirements for administrative access
• Consider temporarily disabling or restricting access to the /manage/dnsmasq/localdomains/ functionality until a patch is applied

Patch Information

As of the last update on 2026-04-02, no official patch has been confirmed in the available CVE data. Organizations should monitor the Endian Community Help Section for security updates and patch announcements. Contact Endian support directly for guidance on remediation and any available hotfixes.

Workarounds

• Implement strict Content Security Policy headers to block inline script execution (script-src 'self')
• Deploy a reverse proxy or WAF in front of the Endian Firewall management interface with XSS filtering enabled
• Limit the number of users with access to the management interface and enforce strong authentication mechanisms
• Regularly audit the remark fields in local domain configurations for suspicious content

# Example: Restrict management interface access via iptables
# Only allow access from trusted admin network
iptables -A INPUT -p tcp --dport 10443 -s 192.168.1.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 10443 -j DROP