CVE-2020-12819
📋 TL;DR
A heap-based buffer overflow vulnerability in FortiGate's SSL VPN daemon allows remote attackers with valid SSL VPN credentials to crash the service by sending a specially crafted Link Control Protocol packet. This affects FortiGate versions 5.6.12, 6.0.10, 6.2.4, 6.4.1 and earlier when tunnel mode is enabled. While arbitrary code execution is theoretically possible, it's considered very difficult to achieve in practice.
💻 Affected Systems
- FortiGate
📦 What is this software?
Fortios by Fortinet
Fortios by Fortinet
Fortios by Fortinet
Fortios by Fortinet
⚠️ Risk & Real-World Impact
Worst Case
Remote code execution leading to complete system compromise, though this is considered extremely difficult to achieve in practice.
Likely Case
Denial of service through SSL VPN daemon crash, disrupting VPN connectivity for legitimate users.
If Mitigated
Minimal impact if patched or if VPN access is restricted to trusted networks only.
🎯 Exploit Status
Exploitation requires valid SSL VPN credentials and specific tunnel mode configuration. Code execution is considered theoretically possible but practically very difficult.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: FortiOS 5.6.13, 6.0.11, 6.2.5, 6.4.2 and later
Vendor Advisory: https://fortiguard.com/advisory/FG-IR-20-082
Restart Required: Yes
Instructions:
1. Log into FortiGate admin interface. 2. Navigate to System > Firmware. 3. Download and install the patched firmware version. 4. Reboot the device after installation completes.
🔧 Temporary Workarounds
Disable SSL VPN tunnel mode
allSwitch SSL VPN from tunnel mode to web mode or disable SSL VPN entirely if not required.
config vpn ssl settings
set tunnel-mode disable
end
Restrict VPN access
allLimit SSL VPN access to specific trusted IP addresses or networks only.
config firewall address
edit "VPN_Trusted_Networks"
set subnet 192.168.1.0 255.255.255.0
next
config vpn ssl settings
set source-address "VPN_Trusted_Networks"
end
🧯 If You Can't Patch
- Implement strict network segmentation to isolate VPN traffic
- Enable logging and monitoring for VPN connection attempts and crashes
🔍 How to Verify
Check if Vulnerable:
Check FortiOS version via CLI: get system status | grep Version. If version is 5.6.12 or earlier, 6.0.10 or earlier, 6.2.4 or earlier, or 6.4.1 or earlier, the system is vulnerable.Check Version:
get system status | grep VersionVerify Fix Applied:
After patching, verify version is 5.6.13+, 6.0.11+, 6.2.5+, or 6.4.2+ using: get system status | grep Version📡 Detection & Monitoring
Log Indicators:
- SSL VPN daemon crash logs
- Unexpected VPN disconnections
- Failed VPN authentication attempts
Network Indicators:
- Large LCP packets to VPN port (typically 443)
- Multiple VPN connection attempts from single source
SIEM Query:
source="fortigate" ("sslvpnd" AND "crash") OR ("VPN" AND "disconnect" AND "unexpected")