CVE-2020-12820
📋 TL;DR
A stack-based buffer overflow vulnerability in FortiOS SSL VPN under non-default configurations allows authenticated remote attackers to crash the FortiClient NAC daemon (fcnacd) and potentially execute arbitrary code by requesting a large FortiClient file name. This affects FortiOS versions 6.0.10 and below, and 5.6.12 and below. Organizations using affected FortiOS versions with SSL VPN enabled are at risk.
💻 Affected Systems
- FortiOS
📦 What is this software?
Fortios by Fortinet
Fortios by Fortinet
⚠️ Risk & Real-World Impact
Worst Case
Remote authenticated attacker gains arbitrary code execution on the FortiOS device, potentially leading to full system compromise, lateral movement, and persistent access.
Likely Case
Remote authenticated attacker crashes the fcnacd daemon, causing denial of service for FortiClient NAC functionality and potentially disrupting SSL VPN operations.
If Mitigated
Minimal impact if SSL VPN is disabled or proper access controls restrict authenticated access to trusted users only.
🎯 Exploit Status
Requires authenticated access to SSL VPN. No public proof-of-concept for arbitrary code execution exists according to vendor.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: FortiOS 6.0.11 and above, 5.6.13 and above
Vendor Advisory: https://fortiguard.fortinet.com/psirt/FG-IR-20-083
Restart Required: Yes
Instructions:
1. Backup current configuration. 2. Download and install FortiOS 6.0.11+ or 5.6.13+ from Fortinet support portal. 3. Apply patch via CLI or web interface. 4. Reboot device after installation. 5. Verify version and functionality.
🔧 Temporary Workarounds
Disable SSL VPN
allTemporarily disable SSL VPN functionality if not required.
config vpn ssl settings
set status disable
end
Restrict SSL VPN Access
allLimit SSL VPN access to trusted IP ranges and users only.
config vpn ssl settings
set source-interface <trusted_interface>
set source-address <trusted_ip_range>
end
🧯 If You Can't Patch
- Disable SSL VPN entirely if not required for business operations.
- Implement strict network segmentation to isolate FortiOS devices from untrusted networks.
🔍 How to Verify
Check if Vulnerable:
Check FortiOS version via CLI: 'get system status' and verify if running 6.0.10 or below, or 5.6.12 or below. Also check if SSL VPN is enabled: 'config vpn ssl settings' and verify status.Check Version:
get system status | grep VersionVerify Fix Applied:
After patching, verify version is 6.0.11+ or 5.6.13+ using 'get system status'. Test SSL VPN functionality to ensure it operates correctly.📡 Detection & Monitoring
Log Indicators:
- Multiple failed fcnacd process restarts in system logs
- Large file name requests in SSL VPN logs
- Unexpected crashes of fcnacd daemon
Network Indicators:
- Unusual SSL VPN traffic patterns with large request sizes
- Multiple connection attempts to fcnacd service from single source
SIEM Query:
source="fortios" AND (process="fcnacd" AND event="crash") OR (protocol="ssl-vpn" AND request_size>threshold)