CVE-2020-23520
📋 TL;DR
CVE-2020-23520 is an authenticated file upload vulnerability in imcat 5.2 that allows attackers to upload malicious files and achieve remote code execution through the picture functionality. This affects any system running imcat 5.2 with authenticated user access. Attackers can compromise the entire system if they gain authenticated access.
💻 Affected Systems
- imcat
📦 What is this software?
Imcat by Txjia
⚠️ Risk & Real-World Impact
Worst Case
Complete system compromise with attacker gaining full control over the server, data exfiltration, lateral movement, and persistent backdoor installation.
Likely Case
Webshell upload leading to data theft, defacement, or use as a pivot point for further attacks within the network.
If Mitigated
Limited impact with proper file upload restrictions and web application firewalls blocking malicious uploads.
🎯 Exploit Status
Exploitation requires authenticated access but is straightforward once credentials are obtained. Public GitHub issues demonstrate the vulnerability.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 5.3 or later
Vendor Advisory: https://github.com/peacexie/imcat/issues/4
Restart Required: No
Instructions:
1. Upgrade imcat to version 5.3 or later. 2. Replace all imcat files with the updated version. 3. Verify the picture upload functionality now properly validates file types.
🔧 Temporary Workarounds
Restrict file upload types
allConfigure web server or application to only allow specific image file extensions (jpg, png, gif) and block executable file types.
# Configure in web server (Apache example):
<Location "/imcat/upload/">
SetEnvIf Request_URI "\.(php|exe|sh|bat)$" block_upload
Deny from env=block_upload
</Location>
Implement file content validation
allAdd server-side validation to check file magic numbers/headers rather than just extensions.
# PHP example for image validation:
function validateImage($file) {
$allowed_types = array(IMAGETYPE_JPEG, IMAGETYPE_PNG, IMAGETYPE_GIF);
$detected_type = exif_imagetype($file['tmp_name']);
return in_array($detected_type, $allowed_types);
}
🧯 If You Can't Patch
- Disable picture upload functionality completely in imcat configuration
- Implement strict network segmentation to isolate imcat servers and limit lateral movement
🔍 How to Verify
Check if Vulnerable:
Check if imcat version is 5.2 by examining version files or configuration. Test if authenticated users can upload non-image files (like .php) through picture upload.Check Version:
Check imcat configuration files or look for version.txt in imcat directory: cat /path/to/imcat/root/version.txtVerify Fix Applied:
After upgrade, attempt to upload a test PHP file through picture upload - it should be rejected. Verify version shows 5.3 or later.📡 Detection & Monitoring
Log Indicators:
- Unusual file uploads to picture functionality
- Uploads of non-image file types (.php, .exe, .sh)
- Multiple failed upload attempts followed by successful suspicious upload
Network Indicators:
- POST requests to upload endpoints with executable file content
- Subsequent outbound connections from imcat server to unknown IPs
SIEM Query:
source="imcat_logs" AND (url_path="/upload/" OR file_extension IN ("php", "exe", "sh", "bat"))