CVE-2021-43406
📋 TL;DR
CVE-2021-43406 is an input validation vulnerability in FusionPBX where the fax_post_size parameter accepts risky characters instead of being constrained to preset values. This allows attackers to potentially inject malicious input that could lead to remote code execution or other attacks. All FusionPBX installations before version 4.5.30 are affected.
💻 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 theft, or deployment of ransomware.
Likely Case
Server-side request forgery, file inclusion, or limited command injection depending on how the input is processed.
If Mitigated
Input validation errors or application crashes if proper input sanitization is implemented.
🎯 Exploit Status
The commit shows input validation was added, suggesting injection attacks were possible. No public exploit code has been identified.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 4.5.30
Vendor Advisory: https://github.com/fusionpbx/fusionpbx/commit/0377b2152c0e59c8f35297f9a9b6ee335a62d963
Restart Required: No
Instructions:
1. Backup your FusionPBX installation and database. 2. Update to FusionPBX version 4.5.30 or later. 3. Verify the update completed successfully.
🔧 Temporary Workarounds
Input Validation Filter
allImplement custom input validation for fax_post_size parameter to restrict to numeric values only.
# Add input validation in relevant PHP files to restrict fax_post_size to numeric values
🧯 If You Can't Patch
- Implement web application firewall (WAF) rules to block requests with non-numeric fax_post_size values.
- Restrict network access to FusionPBX administration interface to trusted IP addresses only.
🔍 How to Verify
Check if Vulnerable:
Check FusionPBX version in admin interface or via command line: grep 'version' /var/www/fusionpbx/resources/config.phpCheck Version:
grep "version" /var/www/fusionpbx/resources/config.phpVerify Fix Applied:
Verify version is 4.5.30 or higher and check that fax_post_size input validation is implemented.📡 Detection & Monitoring
Log Indicators:
- Unusual POST requests to fax-related endpoints with non-standard fax_post_size values
- Application errors related to input validation
Network Indicators:
- HTTP requests with suspicious characters in fax_post_size parameter
SIEM Query:
source="fusionpbx.log" AND (fax_post_size NOT regex "^[0-9]+$")