CVE-2020-21787
📋 TL;DR
CRMEB versions 3.1.0+ contain an unrestricted file upload vulnerability in the UploadService.php component that allows attackers to upload malicious files and execute arbitrary code on the server. This affects all systems running vulnerable CRMEB installations, particularly those with internet-facing web interfaces.
💻 Affected Systems
- CRMEB
📦 What is this software?
Crmeb by Crmeb
⚠️ Risk & Real-World Impact
Worst Case
Complete system compromise leading to data theft, ransomware deployment, or creation of persistent backdoors.
Likely Case
Webshell installation allowing unauthorized access, data exfiltration, and lateral movement within the network.
If Mitigated
Limited impact through proper file upload validation and server hardening.
🎯 Exploit Status
Simple HTTP POST request with malicious file upload to vulnerable endpoint.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Check vendor for latest patched version
Vendor Advisory: https://gitee.com/ZhongBangKeJi/CRMEB/issues/I18MGD
Restart Required: No
Instructions:
1. Update to latest CRMEB version. 2. Apply vendor-provided patch for UploadService.php. 3. Validate file upload functionality is properly secured.
🔧 Temporary Workarounds
File Upload Restriction
allImplement strict file type validation and extension filtering
Modify UploadService.php to validate file extensions and MIME types
Web Server Configuration
linuxRestrict execution permissions in upload directories
chmod 644 /path/to/upload/directory/*
Add 'php_flag engine off' to .htaccess in upload directory
🧯 If You Can't Patch
- Implement WAF rules to block malicious file uploads
- Disable file upload functionality entirely if not required
🔍 How to Verify
Check if Vulnerable:
Test file upload functionality at /crmeb/crmeb/services/UploadService.php with various file typesCheck Version:
Check CRMEB version in configuration files or admin panelVerify Fix Applied:
Attempt to upload malicious files and verify they are rejected or properly sanitized📡 Detection & Monitoring
Log Indicators:
- Unusual file uploads to UploadService.php
- POST requests with executable file extensions
- Webshell access patterns
Network Indicators:
- HTTP POST requests to vulnerable endpoint with file uploads
- Subsequent connections to uploaded malicious files
SIEM Query:
source="web_logs" AND uri="/crmeb/crmeb/services/UploadService.php" AND method="POST" AND (file_extension="php" OR file_extension="jsp" OR file_extension="asp")