CVE-2021-41016
📋 TL;DR
This vulnerability allows authenticated attackers to execute arbitrary shell commands with elevated privileges on Fortinet FortiExtender devices. Attackers can inject malicious commands through CLI inputs containing special characters. Affected users are those running vulnerable FortiExtender versions with authenticated access.
💻 Affected Systems
- Fortinet FortiExtender
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Complete device compromise leading to network foothold, data exfiltration, lateral movement, and persistent backdoor installation.
Likely Case
Unauthorized administrative access, configuration changes, credential harvesting, and potential use as pivot point for further attacks.
If Mitigated
Limited impact due to network segmentation, strong authentication controls, and restricted CLI access.
🎯 Exploit Status
Exploitation requires authenticated access but command injection is straightforward once authenticated.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: FortiExtender 7.0.2, 4.2.4, 4.1.8 or later
Vendor Advisory: https://fortiguard.com/advisory/FG-IR-21-148
Restart Required: Yes
Instructions:
1. Download appropriate firmware from Fortinet support portal. 2. Backup current configuration. 3. Upload firmware via web GUI or CLI. 4. Reboot device. 5. Verify version update.
🔧 Temporary Workarounds
Restrict CLI Access
allLimit CLI access to trusted administrators only and implement strong authentication controls.
Network Segmentation
allIsolate FortiExtender management interfaces from untrusted networks.
🧯 If You Can't Patch
- Implement strict access controls and multi-factor authentication for all administrative accounts.
- Monitor CLI command logs for unusual patterns and implement network segmentation to limit blast radius.
🔍 How to Verify
Check if Vulnerable:
Check FortiExtender firmware version via web GUI (System > Dashboard) or CLI command 'get system status'.Check Version:
get system status | grep VersionVerify Fix Applied:
Verify firmware version is 7.0.2+, 4.2.4+, or 4.1.8+ after update.📡 Detection & Monitoring
Log Indicators:
- Unusual CLI command patterns
- Multiple failed authentication attempts followed by successful login
- Commands containing special characters like ;, |, &, $, (, )
Network Indicators:
- Unexpected outbound connections from FortiExtender
- Unusual traffic patterns to/from management interfaces
SIEM Query:
source="fortiextender" AND (command="*;*" OR command="*|*" OR command="*&*" OR command="*$(*)*")