CVE-2020-24186: Gvectors wpDiscuz RCE Vulnerability

CVE-2020-24186 Overview

A critical Remote Code Execution vulnerability exists in the gVectors wpDiscuz plugin versions 7.0 through 7.0.4 for WordPress. This vulnerability allows unauthenticated users to upload arbitrary files, including malicious PHP scripts, through the wmuUploadFiles AJAX action. Successful exploitation grants attackers complete control over the affected WordPress installation and potentially the underlying server.

Critical Impact

Unauthenticated attackers can achieve full server compromise by uploading and executing malicious PHP files, leading to complete website takeover, data theft, and lateral movement within the hosting environment.

Affected Products

• gVectors wpDiscuz 7.0 for WordPress
• gVectors wpDiscuz 7.0.1 through 7.0.3 for WordPress
• gVectors wpDiscuz 7.0.4 for WordPress

Discovery Timeline

• 2020-08-24 - CVE-2020-24186 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-24186

Vulnerability Analysis

This vulnerability is classified as CWE-434 (Unrestricted Upload of File with Dangerous Type). The wpDiscuz plugin implements a comment system for WordPress that includes functionality allowing users to attach files to their comments. The vulnerability resides in the file upload handling mechanism of the wmuUploadFiles AJAX action.

The core issue stems from insufficient file type validation during the upload process. While the plugin attempts to restrict file uploads to safe types, the implementation fails to properly enforce these restrictions. Attackers can bypass the file type checks and upload PHP files disguised as legitimate image files or by exploiting weaknesses in the MIME type detection logic.

Since the AJAX action does not require authentication, any remote attacker can exploit this vulnerability without needing valid credentials. Once a PHP file is uploaded, the attacker can access it directly through the web server, triggering code execution with the privileges of the web server process.

Root Cause

The vulnerability originates from improper input validation in the file upload functionality. The wmuUploadFiles AJAX handler fails to adequately verify that uploaded files are actually safe file types. The plugin relies on client-controllable data for file type determination rather than performing robust server-side validation of file contents. This allows attackers to manipulate the file extension or MIME type headers to bypass the intended restrictions and upload executable PHP code.

Attack Vector

The attack is network-based and requires no authentication or user interaction. An attacker can exploit this vulnerability by sending a crafted HTTP POST request to the WordPress AJAX handler endpoint (/wp-admin/admin-ajax.php) with the action parameter set to wmuUploadFiles. The malicious payload contains a PHP webshell or other malicious code disguised to bypass the plugin's file type checks.

The attack flow typically involves:

1. The attacker identifies a WordPress site running a vulnerable version of wpDiscuz
2. A specially crafted multipart form request is sent to the AJAX endpoint
3. The malicious PHP file is uploaded to the server's upload directory
4. The attacker accesses the uploaded file directly via its URL
5. The PHP code executes, granting the attacker command execution capabilities

Technical details and proof-of-concept exploits are available in the Packet Storm Shell Upload Exploit and the Wordfence Blog Patch Analysis.

Detection Methods for CVE-2020-24186

Indicators of Compromise

• Unexpected PHP files appearing in WordPress upload directories (wp-content/uploads/)
• HTTP POST requests to /wp-admin/admin-ajax.php with action wmuUploadFiles from suspicious IP addresses
• Web server logs showing access to newly created PHP files in upload directories
• Unusual process spawning from the web server user (www-data, apache, nginx)

Detection Strategies

• Monitor WordPress AJAX endpoints for suspicious file upload activity, particularly requests containing the wmuUploadFiles action
• Implement file integrity monitoring on upload directories to detect unauthorized PHP file creation
• Deploy web application firewall (WAF) rules to block requests attempting to upload PHP files through the wpDiscuz upload handler
• Review web server access logs for patterns indicating webshell access or reconnaissance activity

Monitoring Recommendations

• Enable verbose logging on WordPress AJAX handlers and correlate with file system changes
• Configure alerts for any PHP file creation events in wp-content/uploads/ directories
• Monitor for outbound network connections initiated by the web server process, which may indicate successful compromise
• Implement real-time log analysis to detect exploitation attempts against known wpDiscuz endpoints

How to Mitigate CVE-2020-24186

Immediate Actions Required

• Upgrade wpDiscuz plugin to version 7.0.5 or later immediately
• Audit all WordPress upload directories for unauthorized PHP files and remove any suspicious uploads
• Review web server logs for evidence of exploitation attempts or successful compromise
• Consider temporarily disabling the wpDiscuz plugin if immediate patching is not possible

Patch Information

The vulnerability was addressed by gVectors in wpDiscuz version 7.0.5, which implements proper server-side file type validation. Administrators should update through the WordPress plugin dashboard or by downloading the patched version directly from the WordPress plugin repository. For detailed analysis of the vulnerability and patch, refer to the Wordfence Blog Patch Analysis.

Workarounds

• Disable file upload functionality in wpDiscuz settings until the plugin can be updated
• Implement server-level restrictions to prevent PHP execution within upload directories using web server configuration
• Use a WAF to block requests to the wmuUploadFiles AJAX action
• Restrict access to admin-ajax.php from untrusted IP ranges if feasible for your environment

# Apache .htaccess configuration to prevent PHP execution in uploads
# Add to wp-content/uploads/.htaccess
<FilesMatch "\.php$">
    Order Deny,Allow
    Deny from all
</FilesMatch>

# Nginx configuration alternative
# Add to server block
location ~* /wp-content/uploads/.*\.php$ {
    deny all;
}