CVE-2021-40883
📋 TL;DR
This vulnerability allows unauthenticated attackers to upload malicious PHP files through the plugin upload functionality in emlog, leading to remote code execution. It affects all emlog 5.3.1 installations with default configurations. Attackers can gain complete control over affected web servers.
💻 Affected Systems
- emlog
📦 What is this software?
Emlog by Emlog
⚠️ Risk & Real-World Impact
Worst Case
Complete server compromise allowing attackers to execute arbitrary code, steal data, install backdoors, pivot to internal networks, and use the server for further attacks.
Likely Case
Webshell installation leading to data theft, defacement, cryptocurrency mining, or participation in botnets.
If Mitigated
Limited impact with proper file upload restrictions, web application firewalls, and network segmentation in place.
🎯 Exploit Status
Simple file upload bypass exploit with publicly available proof-of-concept code. No authentication required.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 5.3.2 or later
Vendor Advisory: https://github.com/emlog/emlog/issues/108
Restart Required: No
Instructions:
1. Backup your emlog installation and database. 2. Download emlog 5.3.2 or later from the official repository. 3. Replace all files except config.php and upload folder. 4. Verify the installation works correctly.
🔧 Temporary Workarounds
Disable Plugin Upload
linuxRemove or restrict access to the plugin upload functionality
chmod 000 content/plugins/
rm -rf content/plugins/upload.php
Web Application Firewall Rules
allBlock requests to plugin upload endpoints
🧯 If You Can't Patch
- Implement strict file upload validation and whitelisting for allowed file types
- Deploy a web application firewall with rules blocking PHP file uploads to plugin directories
🔍 How to Verify
Check if Vulnerable:
Check if emlog version is 5.3.1 by examining the version.php file or admin panelCheck Version:
grep "define('EMLOG_VERSION'" version.phpVerify Fix Applied:
Verify version is 5.3.2 or later and test that plugin upload functionality properly validates file types📡 Detection & Monitoring
Log Indicators:
- File upload attempts to content/plugins/ directory
- POST requests with .php files to upload endpoints
- Unusual file creation in plugin directories
Network Indicators:
- HTTP POST requests with multipart/form-data containing PHP files
- Traffic to unexpected plugin upload endpoints
SIEM Query:
source="web_logs" AND (uri_path="/content/plugins/" OR file_extension=".php") AND http_method="POST"