CVE-2020-37023: Koken CMS File Upload RCE Vulnerability

CVE-2020-37023 Overview

CVE-2020-37023 is a file upload vulnerability in Koken CMS version 0.22.24 that allows authenticated attackers to bypass file extension restrictions by renaming malicious PHP files. This vulnerability enables attackers to upload PHP files with system command execution capabilities by manipulating the file upload request through a web proxy and changing the file extension, potentially leading to complete server compromise.

Critical Impact

Authenticated attackers can achieve remote code execution by uploading malicious PHP files that execute system commands on the underlying server.

Affected Products

• Koken CMS version 0.22.24

Discovery Timeline

• 2026-01-30 - CVE-2020-37023 published to NVD
• 2026-02-04 - Last updated in NVD database

Technical Details for CVE-2020-37023

Vulnerability Analysis

This vulnerability (CWE-434: Unrestricted Upload of File with Dangerous Type) exists due to insufficient server-side validation of uploaded files in Koken CMS. While the application implements file extension restrictions on the client side or through basic server-side checks, these controls can be easily circumvented by an authenticated attacker using a web proxy to intercept and modify the upload request.

The attack requires valid authentication credentials to access the file upload functionality. Once authenticated, an attacker can leverage tools like Burp Suite to intercept the HTTP request during file upload and modify the file extension from a permitted format (such as .jpg) to a dangerous format like .php. The server fails to properly validate the file type after this modification, allowing the malicious file to be stored and subsequently executed.

Root Cause

The root cause of this vulnerability lies in the application's reliance on client-side validation and insufficient server-side verification of uploaded file types. The application does not perform adequate content-type verification, magic byte checking, or proper extension validation after the file is received by the server. This allows attackers to bypass the intended file upload restrictions by modifying requests in transit.

Attack Vector

The attack is network-based and requires the attacker to have valid authentication credentials for the Koken CMS administrative interface. The exploitation process involves:

1. Authenticating to the Koken CMS admin panel
2. Initiating a file upload with a legitimate file extension
3. Intercepting the HTTP request using a web proxy
4. Modifying the filename extension to .php while keeping malicious PHP code as the file content
5. Forwarding the modified request to the server
6. Accessing the uploaded PHP file directly to execute arbitrary commands

The vulnerability mechanism involves manipulating HTTP multipart form data. When an attacker intercepts the file upload request, they modify the filename parameter in the Content-Disposition header from a safe extension to .php. The server accepts this modified request and stores the file with the PHP extension, making it executable. For detailed technical information, refer to the GitHub PoC for Koken CMS and Exploit-DB #48706.

Detection Methods for CVE-2020-37023

Indicators of Compromise

• Unexpected PHP files appearing in upload directories, particularly with names resembling image files
• Web server access logs showing requests to PHP files in media or upload directories
• Suspicious POST requests to file upload endpoints with modified Content-Type or filename parameters
• Server-side command execution artifacts such as unusual process spawning from web server processes

Detection Strategies

• Monitor file upload directories for files with executable extensions (.php, .phtml, .php5)
• Implement web application firewall (WAF) rules to detect file extension manipulation in multipart form uploads
• Review web server access logs for requests to PHP files in directories typically reserved for media uploads
• Deploy file integrity monitoring on upload directories to detect unauthorized file creation

Monitoring Recommendations

• Enable verbose logging for Koken CMS file upload operations
• Configure alerts for any PHP file creation in upload/media directories
• Monitor for web shell indicators such as base64 encoding functions, system(), exec(), or passthru() calls in uploaded content
• Implement network monitoring for suspicious outbound connections from web server processes

How to Mitigate CVE-2020-37023

Immediate Actions Required

• Restrict access to the Koken CMS administrative interface to trusted IP addresses only
• Implement additional authentication controls such as two-factor authentication for admin accounts
• Review and remove any suspicious PHP files from upload directories
• Consider disabling file upload functionality until a patch is applied or workarounds are implemented

Patch Information

As of the last update, Koken CMS development has been discontinued and no official patch is available for this vulnerability. Organizations using Koken CMS should consider migrating to an actively maintained content management system. For additional context, refer to the VulnCheck Advisory for Koken CMS and the Koken CMS Homepage.

Workarounds

• Configure the web server to prevent PHP execution in upload directories by adding php_flag engine off in the relevant .htaccess file
• Implement server-level file upload validation that checks magic bytes and rejects files with executable extensions
• Use a reverse proxy or WAF to inspect and block file upload requests containing suspicious content or extension manipulation attempts
• Restrict file permissions on upload directories to prevent execution of uploaded files

# Apache configuration to disable PHP execution in upload directories
# Add to .htaccess in the upload directory or virtual host configuration
<Directory "/path/to/koken/storage/uploads">
    php_flag engine off
    <FilesMatch "\.(php|phtml|php5|php7)$">
        Require all denied
    </FilesMatch>
</Directory>