CVE-2020-28871: Monitorr RCE Vulnerability via Upload

CVE-2020-28871 Overview

CVE-2020-28871 is a critical remote code execution (RCE) vulnerability affecting Monitorr version 1.7.6m. The vulnerability exists in the upload.php file, which allows an unauthorized attacker to execute arbitrary code on the server-side by exploiting an insecure file upload mechanism. This flaw enables attackers to upload malicious files, such as web shells, without authentication, potentially leading to complete system compromise.

Critical Impact

Unauthenticated attackers can achieve remote code execution on vulnerable Monitorr servers through the insecure file upload functionality, potentially leading to full system compromise, data theft, and lateral movement within the network.

Affected Products

• Monitorr version 1.7.6m
• Monitorr version 1.7.7d (confirmed affected in exploit reports)

Discovery Timeline

• 2021-02-10 - CVE-2020-28871 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-28871

Vulnerability Analysis

This vulnerability is classified as CWE-434 (Unrestricted Upload of File with Dangerous Type). The upload.php script in Monitorr fails to properly validate or restrict the types of files that can be uploaded by users. The vulnerability is particularly severe because it requires no authentication, meaning any network-accessible attacker can exploit it without needing credentials or prior access to the application.

The insecure file upload implementation allows attackers to bypass intended security controls and upload executable files such as PHP web shells directly to the server. Once uploaded, the attacker can access the malicious file through the web server to execute arbitrary commands with the privileges of the web server user.

Root Cause

The root cause of this vulnerability is the lack of proper input validation and file type restrictions in the upload.php endpoint. The application fails to:

1. Verify that the uploaded file conforms to expected file types (e.g., images only)
2. Sanitize file names to prevent path traversal or extension manipulation
3. Implement authentication checks before allowing file uploads
4. Store uploaded files outside of web-accessible directories or without executable permissions

Attack Vector

The attack is network-based and requires no user interaction or authentication. An attacker can exploit this vulnerability by sending a crafted HTTP POST request to the upload.php endpoint containing a malicious file disguised or uploaded directly as a PHP script.

The exploitation typically follows this pattern:

1. The attacker identifies a vulnerable Monitorr installation accessible over the network
2. A malicious PHP file (typically a web shell) is uploaded via the upload.php endpoint
3. The attacker accesses the uploaded file through a predictable URL path
4. Commands are executed on the server with web server user privileges

Multiple public exploits have been documented for this vulnerability, including shell upload and bypass techniques. For detailed technical analysis, refer to the Lyhins Lab Analysis on Monitorr Exploits or the Exploit-DB #48980 entry.

Detection Methods for CVE-2020-28871

Indicators of Compromise

• Unexpected PHP files appearing in Monitorr upload directories
• Web server logs showing POST requests to upload.php from external IP addresses
• Presence of web shell files with suspicious names or encoded content
• Unusual outbound connections from the web server process
• Process spawning from web server processes (e.g., /bin/sh or cmd.exe child processes of Apache/Nginx)

Detection Strategies

• Monitor web server logs for suspicious POST requests to /assets/php/upload.php or similar upload endpoints
• Implement file integrity monitoring (FIM) on web application directories to detect unauthorized file additions
• Deploy web application firewalls (WAF) configured to inspect file upload content and block executable scripts
• Use endpoint detection solutions to identify shell execution patterns originating from web server processes

Monitoring Recommendations

• Enable verbose logging on web servers hosting Monitorr installations
• Configure SIEM rules to alert on file upload activity to Monitorr applications followed by web shell indicators
• Monitor for command execution patterns consistent with web shell activity
• Implement network monitoring to detect reverse shell connections or unusual egress traffic from web servers

How to Mitigate CVE-2020-28871

Immediate Actions Required

• Immediately remove or restrict network access to vulnerable Monitorr installations
• Audit upload directories for any suspicious or unknown files and remove them
• Review web server logs for evidence of exploitation attempts
• Consider disabling the upload functionality entirely if not required
• Isolate affected systems from critical network segments until patched

Patch Information

Organizations should upgrade to a patched version of Monitorr that addresses the insecure file upload vulnerability. No official vendor advisory was available in the CVE data. System administrators should check the official Monitorr repository or project page for the latest security updates and apply them immediately.

Workarounds

• Restrict access to the upload.php file using web server access controls (e.g., .htaccess for Apache or location blocks for Nginx)
• Implement additional authentication in front of the Monitorr application using a reverse proxy
• Configure the web server to deny execution of PHP files in upload directories
• Use network-level controls (firewall rules, VPN) to limit access to Monitorr to trusted IP addresses only
• Consider deploying a web application firewall (WAF) to filter malicious upload attempts

# Apache configuration example - Deny PHP execution in uploads directory
<Directory "/var/www/monitorr/assets/data/usrimg">
    <FilesMatch "\.php$">
        Require all denied
    </FilesMatch>
</Directory>

# Nginx configuration example - Block PHP execution in uploads
location ~* /assets/data/usrimg/.*\.php$ {
    deny all;
}