CVE-2024-5441: Modern Events Calendar RCE Vulnerability

CVE-2024-5441 Overview

CVE-2024-5441 is an arbitrary file upload vulnerability affecting the Modern Events Calendar plugin for WordPress developed by Webnus. The vulnerability exists due to missing file type validation in the set_featured_image function in all versions up to and including 7.11.0. This security flaw allows authenticated attackers with subscriber-level access or higher to upload arbitrary files to the affected site's server, potentially leading to remote code execution.

The risk is further amplified by a plugin configuration option that allows administrators to extend event submission capabilities to unauthenticated users. When this setting is enabled, even unauthenticated attackers can exploit this vulnerability, significantly expanding the attack surface.

Critical Impact

Attackers can upload malicious files including web shells to achieve remote code execution on vulnerable WordPress installations, potentially leading to complete site compromise.

Affected Products

• Webnus Modern Events Calendar (versions up to and including 7.11.0)
• Webnus Modern Events Calendar Lite (versions up to and including 7.11.0)
• WordPress sites with the Modern Events Calendar plugin installed

Discovery Timeline

• 2024-07-09 - CVE-2024-5441 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2024-5441

Vulnerability Analysis

This vulnerability falls under CWE-434 (Unrestricted Upload of File with Dangerous Type). The core issue stems from the plugin's failure to properly validate file types when processing image uploads through the set_featured_image function. This function is intended to handle featured image uploads for events but lacks essential security controls to restrict uploaded file types.

The vulnerability enables attackers to bypass intended restrictions and upload executable files such as PHP web shells. Once uploaded, these malicious files can be accessed directly through the web server, allowing attackers to execute arbitrary code within the context of the web application. The network-based attack vector requires low attack complexity and no user interaction, making it highly exploitable in real-world scenarios.

Root Cause

The root cause of this vulnerability is the absence of server-side file type validation in the set_featured_image function. The plugin relies on insufficient or non-existent checks to verify that uploaded files are legitimate image formats before storing them on the server. Without proper MIME type validation, file extension whitelisting, or content inspection, the function accepts any file type submitted through the upload mechanism.

Additionally, the plugin's architectural decision to allow administrators to extend event submission to unauthenticated users creates a pathway for anonymous exploitation, transforming what could be a post-authentication vulnerability into a potentially unauthenticated attack vector.

Attack Vector

The attack leverages the event submission functionality within the Modern Events Calendar plugin. An attacker can craft a malicious request to the set_featured_image function, substituting a legitimate image file with a malicious PHP script disguised with an image-related filename or submitted directly as an executable file.

The attack flow typically involves:

1. Identifying a WordPress site running a vulnerable version of Modern Events Calendar
2. Authenticating with subscriber-level credentials (or exploiting unauthenticated access if enabled)
3. Intercepting or crafting a request to upload a featured image for an event
4. Replacing the image payload with a malicious PHP web shell
5. Accessing the uploaded file directly via its URL to execute arbitrary commands

For more technical details on the exploitation mechanism, refer to the Wordfence Vulnerability Analysis.

Detection Methods for CVE-2024-5441

Indicators of Compromise

• Unexpected PHP or executable files in WordPress upload directories, particularly in paths associated with the Modern Events Calendar plugin
• Unusual HTTP POST requests to event submission endpoints containing non-image file content
• Web shell signatures in uploaded files within the /wp-content/uploads/ directory
• Suspicious outbound connections originating from the web server process
• Anomalous file creation activity in plugin-related directories

Detection Strategies

• Monitor file upload endpoints for requests containing executable file extensions or suspicious MIME types
• Implement Web Application Firewall (WAF) rules to detect and block file upload attempts with dangerous extensions
• Utilize file integrity monitoring to detect unauthorized file additions in WordPress directories
• Review web server access logs for direct access to unusual files in upload directories
• Deploy endpoint detection solutions like SentinelOne to identify web shell behavior and post-exploitation activity

Monitoring Recommendations

• Enable detailed logging for WordPress file upload activities and review regularly
• Configure alerts for new file creation events in the WordPress uploads directory
• Monitor for PHP execution from upload directories which should not normally execute server-side code
• Implement network traffic analysis to detect command-and-control communications from compromised servers
• Use SentinelOne's behavioral AI engine to detect and respond to web shell deployment and execution patterns

How to Mitigate CVE-2024-5441

Immediate Actions Required

• Update the Modern Events Calendar plugin to a version newer than 7.11.0 immediately
• Audit WordPress upload directories for suspicious files, particularly PHP scripts in image folders
• Disable the "Allow unauthenticated event submission" setting if enabled until the plugin is patched
• Review user accounts with subscriber-level access or higher for any unauthorized additions
• Implement file upload restrictions at the web server level as an additional defense layer

Patch Information

Webnus has addressed this vulnerability in versions released after 7.11.0 of the Modern Events Calendar plugin. Site administrators should update to the latest available version through the WordPress plugin repository or directly from the Webnus Modern Events Calendar official website.

Before updating, ensure you have a complete backup of your WordPress installation. After updating, verify the plugin version in the WordPress admin dashboard and confirm that the set_featured_image function now includes proper file type validation.

Workarounds

• Restrict event submission functionality to trusted administrator accounts only until the patch is applied
• Implement server-level file upload filtering using .htaccess rules or web server configuration to block execution of PHP files in upload directories
• Use a Web Application Firewall (WAF) to inspect and filter malicious file upload attempts
• Consider temporarily deactivating the Modern Events Calendar plugin if immediate patching is not possible
• Deploy SentinelOne endpoint protection to detect and prevent exploitation attempts and web shell activity

# Apache .htaccess configuration to prevent PHP execution in uploads directory
# Add to /wp-content/uploads/.htaccess
<FilesMatch "\.(?:php|phtml|php3|php4|php5|php7|phps)$">
    Require all denied
</FilesMatch>

# Alternative: Disable script execution entirely in uploads
<Directory "/path/to/wordpress/wp-content/uploads">
    php_admin_flag engine off
</Directory>