CVE-2021-47753: phpKF CMS RCE Vulnerability

CVE-2021-47753 Overview

CVE-2021-47753 is an unauthenticated file upload vulnerability affecting phpKF CMS 3.00 Beta y6. This remote code execution flaw allows attackers to bypass file extension validation checks and upload malicious PHP files disguised as image files. Once uploaded, attackers can rename the file to restore its PHP extension and execute arbitrary system commands through a crafted web shell parameter, effectively gaining complete control over the vulnerable web server.

Critical Impact

Unauthenticated remote code execution allowing attackers to upload and execute arbitrary PHP code, leading to complete server compromise without requiring any credentials.

Affected Products

• phpKF CMS 3.00 Beta y6

Discovery Timeline

• 2026-01-15 - CVE CVE-2021-47753 published to NVD
• 2026-01-16 - Last updated in NVD database

Technical Details for CVE-2021-47753

Vulnerability Analysis

This vulnerability is classified as CWE-434 (Unrestricted Upload of File with Dangerous Type). The phpKF CMS file upload functionality fails to properly validate uploaded files, relying solely on file extension checks that can be trivially bypassed. The attack surface is exposed over the network and requires no authentication or user interaction, making it particularly dangerous for internet-facing deployments.

The exploitation chain involves three stages: first, the attacker uploads a PHP web shell with a .png extension to bypass the initial extension filter; second, the attacker leverages a file rename functionality to change the extension back to .php; and finally, the attacker accesses the renamed file directly to execute arbitrary commands via a parameter passed to the web shell.

Root Cause

The root cause of this vulnerability lies in the inadequate file upload validation mechanism within phpKF CMS. The application performs file type validation based solely on the file extension rather than examining the actual file content (magic bytes) or MIME type. Additionally, the CMS provides functionality that allows uploaded files to be renamed, enabling attackers to restore malicious extensions post-upload. This combination of weak validation and unrestricted rename capabilities creates a complete bypass of the intended security controls.

Attack Vector

The attack vector is network-based, requiring no authentication or prior privileges. An attacker can exploit this vulnerability remotely by:

1. Crafting a PHP web shell payload containing command execution functionality
2. Renaming the malicious .php file to use a .png extension
3. Uploading the disguised file through the vulnerable upload endpoint
4. Using the CMS rename functionality to change the extension back to .php
5. Accessing the uploaded PHP file directly via HTTP and passing commands through the web shell parameter

This attack can be performed entirely through standard HTTP requests, making it accessible to attackers with basic knowledge of web application exploitation. The Exploit-DB #50610 reference provides additional technical details about this exploitation technique.

Detection Methods for CVE-2021-47753

Indicators of Compromise

• Presence of unexpected PHP files in upload directories, particularly those recently renamed from image extensions
• Web server access logs showing direct requests to files in upload directories with query parameters suggestive of command execution (e.g., cmd=, c=, exec=)
• Unusual file rename operations in CMS activity logs, especially extensions being changed from image formats to .php
• Newly created PHP files with characteristics of web shells (small size, obfuscated content, eval(), system(), or shell_exec() functions)

Detection Strategies

• Implement file integrity monitoring on web server upload directories to detect unauthorized PHP files or unexpected extension changes
• Configure web server access log analysis to alert on direct access to files in upload directories, especially with query string parameters
• Deploy web application firewall (WAF) rules to detect and block common web shell patterns in uploaded file content
• Monitor for process spawning from web server processes (e.g., www-data spawning shell commands)

Monitoring Recommendations

• Enable detailed logging of all file upload and rename operations within the CMS
• Configure real-time alerting for any new .php files created in upload directories
• Implement network-level monitoring for outbound connections from the web server that may indicate reverse shell activity
• Review web server error logs for signs of attempted PHP execution in non-standard directories

How to Mitigate CVE-2021-47753

Immediate Actions Required

• Remove or disable phpKF CMS 3.00 Beta y6 from production environments until a patched version is available
• Implement web server configuration to prevent PHP execution in upload directories
• Review upload directories for any suspicious files and remove unauthorized content
• Apply network segmentation to limit the impact of potential compromise
• Monitor the phpKF official website for security updates

Patch Information

At the time of publication, no official vendor patch has been identified for this vulnerability. Organizations using phpKF CMS 3.00 Beta y6 should check the phpKF download section for updated versions that may address this issue. Consider migrating to a actively maintained CMS platform if updates are not forthcoming.

Workarounds

• Configure the web server to deny PHP execution in upload directories using .htaccess rules or server configuration directives
• Implement content-based file validation that checks magic bytes rather than relying solely on file extensions
• Restrict file upload functionality to authenticated and authorized users only
• Use a Web Application Firewall (WAF) to inspect uploaded file content for malicious patterns
• Consider placing upload directories on a separate domain or server without PHP execution capabilities

# Apache configuration to prevent PHP execution in upload directories
<Directory "/var/www/html/uploads">
    php_flag engine off
    <FilesMatch "\.php$">
        Require all denied
    </FilesMatch>
</Directory>

# Nginx configuration equivalent
location ~* /uploads/.*\.php$ {
    deny all;
}