CVE-2021-43404
📋 TL;DR
FusionPBX before version 4.5.30 contains a filename sanitization vulnerability in FAX file handling. Attackers can exploit risky characters in FAX filenames to potentially execute arbitrary code or access unauthorized files. This affects all FusionPBX installations running vulnerable versions.
💻 Affected Systems
- FusionPBX
📦 What is this software?
Fusionpbx by Fusionpbx
⚠️ Risk & Real-World Impact
Worst Case
Remote code execution leading to complete system compromise, data exfiltration, or lateral movement within the network.
Likely Case
File system access allowing reading/writing of sensitive files, directory traversal, or limited command execution.
If Mitigated
No impact if proper input validation and file path restrictions are implemented.
🎯 Exploit Status
Exploitation requires access to FAX functionality. The commit suggests filename sanitization issues that could lead to path traversal or command injection.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 4.5.30 and later
Vendor Advisory: https://github.com/fusionpbx/fusionpbx/commit/487afc371e5c0dfbbc07cd002333c5bcd949d0f4
Restart Required: No
Instructions:
1. Backup your FusionPBX configuration and database. 2. Update FusionPBX to version 4.5.30 or later using the upgrade script. 3. Verify the fix by checking the version and testing FAX functionality.
🔧 Temporary Workarounds
Disable FAX functionality
linuxTemporarily disable FAX features until patching is possible
# Edit FusionPBX configuration to disable FAX modules
# Remove or comment FAX-related configurations in /etc/fusionpbx/config.php
Implement file upload restrictions
linuxAdd web server rules to restrict FAX file uploads
# Add to Apache/Nginx configuration:
location ~ \.fax$ { deny all; }
🧯 If You Can't Patch
- Implement strict input validation for all FAX filename inputs
- Deploy WAF rules to block malicious filename patterns and path traversal attempts
🔍 How to Verify
Check if Vulnerable:
Check FusionPBX version: php /var/www/fusionpbx/resources/functions.php | grep versionCheck Version:
php /var/www/fusionpbx/resources/functions.php | grep 'version'Verify Fix Applied:
Confirm version is 4.5.30 or higher and test FAX upload with special characters in filename📡 Detection & Monitoring
Log Indicators:
- Unusual FAX file upload attempts with special characters
- Failed file operations in FAX directories
- Web server errors related to file path resolution
Network Indicators:
- HTTP requests to FAX endpoints with encoded characters in filenames
- Unusual file upload patterns to FAX interfaces
SIEM Query:
source="fusionpbx.log" AND ("fax" AND (".." OR "%" OR "|" OR ";"))