CVE-2020-37113: GUnet OpenEclass 1.7.3 RCE Vulnerability

CVE-2020-37113 Overview

GUnet OpenEclass 1.7.3 contains a file upload extension bypass vulnerability that allows authenticated users to upload malicious files by circumventing file type restrictions. By renaming a PHP file to use alternative extensions such as .php3 or .PhP, an attacker can successfully upload a web shell and execute arbitrary code on the server. This vulnerability specifically affects the exercise submission feature, enabling remote code execution through improper file type validation.

Critical Impact

Authenticated attackers can achieve remote code execution on vulnerable OpenEclass servers by uploading web shells through the exercise submission feature, potentially leading to full server compromise.

Affected Products

• GUnet OpenEclass 1.7.3
• GUnet OpenEclass e-Learning Platform (versions prior to security patch)

Discovery Timeline

• 2026-02-03 - CVE CVE-2020-37113 published to NVD
• 2026-02-04 - Last updated in NVD database

Technical Details for CVE-2020-37113

Vulnerability Analysis

This vulnerability is classified as CWE-434 (Unrestricted Upload of File with Dangerous Type). The file upload mechanism in GUnet OpenEclass 1.7.3 fails to properly validate file extensions using case-insensitive comparison and does not account for alternative PHP executable extensions. When users submit files through the exercise submission feature, the application applies a blocklist-based approach to prevent PHP uploads. However, this validation can be trivially bypassed using case variations (.PhP, .pHp) or alternative PHP extensions (.php3, .php4, .php5, .phtml) that Apache and other web servers may still execute as PHP.

The network-accessible attack vector requires only low privileges (authenticated user access), making exploitation straightforward for any registered user on the platform. Successful exploitation grants attackers the ability to execute arbitrary commands with the privileges of the web server process, potentially leading to complete server compromise, data exfiltration, or lateral movement within the network.

Root Cause

The root cause is insufficient input validation in the file upload handler within the exercise submission functionality. The application uses an inadequate file extension blocklist that:

1. Performs case-sensitive extension matching, allowing bypass via mixed-case extensions like .PhP
2. Does not include all executable PHP file extensions (.php3, .php4, .php5, .phtml)
3. Relies solely on extension checking without validating file content or MIME types

This incomplete validation allows attackers to bypass the intended security controls and upload files that will be executed by the web server as PHP scripts.

Attack Vector

The attack exploits the file upload functionality accessible to authenticated users. An attacker with valid credentials can navigate to the exercise submission feature and upload a malicious PHP file with a modified extension. The web server, configured to execute multiple PHP-related extensions, will process the uploaded file when accessed, executing the embedded malicious code.

The attack flow involves:

1. Authenticating to the OpenEclass platform with valid user credentials
2. Creating or locating a PHP web shell payload
3. Renaming the file to use a bypassed extension (e.g., shell.php3 or shell.PhP)
4. Uploading the renamed file through the exercise submission feature
5. Accessing the uploaded file directly via its URL path to trigger code execution

For detailed technical analysis and proof-of-concept information, refer to the Exploit-DB #48163 entry.

Detection Methods for CVE-2020-37113

Indicators of Compromise

• Presence of PHP files with non-standard extensions (.php3, .php4, .php5, .phtml, .PhP) in exercise upload directories
• Unusual web server access logs showing requests to recently uploaded files with alternative PHP extensions
• Unexpected outbound network connections originating from the web server process
• New or modified files in upload directories that contain PHP code signatures or web shell patterns

Detection Strategies

• Implement file integrity monitoring on OpenEclass upload directories to detect unauthorized file additions
• Configure web application firewall (WAF) rules to block requests for files with alternative PHP extensions in upload paths
• Deploy endpoint detection and response (EDR) solutions to monitor for process spawning from the web server
• Review Apache or web server access logs for patterns indicating web shell access attempts

Monitoring Recommendations

• Enable detailed logging for file upload operations within the OpenEclass application
• Monitor system calls and process creation events from the web server process for suspicious command execution
• Set up alerts for files uploaded with executable extensions or mixed-case PHP extensions
• Implement behavioral analysis to detect anomalous user activity patterns during file submissions

How to Mitigate CVE-2020-37113

Immediate Actions Required

• Upgrade GUnet OpenEclass to a patched version as indicated in the Open eClass Changes Document
• Review and remove any suspicious files with alternative PHP extensions from upload directories
• Implement strict allowlist-based file extension validation at the application level
• Restrict web server configuration to only execute .php files, disabling execution of alternative extensions

Patch Information

Administrators should consult the official Open eClass Homepage for the latest security updates and patched versions. The VulnCheck Advisory for Open eClass provides additional context on affected versions and remediation guidance.

Workarounds

• Configure Apache to only execute files with the exact .php extension in upload directories
• Implement server-side content inspection to validate uploaded files are not executable code
• Move uploaded files outside the web root and serve them through a proxy script that sets appropriate headers
• Apply file permission restrictions to prevent execution of files in upload directories

# Apache configuration to disable PHP execution in upload directories
<Directory "/var/www/openeclass/upload">
    # Disable PHP execution
    php_admin_flag engine off
    
    # Prevent execution of alternative PHP extensions
    <FilesMatch "\.(php[0-9]?|phtml|phar)$">
        Require all denied
    </FilesMatch>
    
    # Force download for all files
    ForceType application/octet-stream
    Header set Content-Disposition attachment
</Directory>