CVE-2021-42756
📋 TL;DR
This critical vulnerability allows unauthenticated remote attackers to execute arbitrary code on FortiWeb web application firewalls via specially crafted HTTP requests. It affects multiple FortiWeb versions across the 5.x, 6.0, 6.1, 6.2, 6.3, and 6.4 branches. Organizations using vulnerable FortiWeb devices as internet-facing proxies are at immediate risk.
💻 Affected Systems
- FortiWeb Web Application Firewall
📦 What is this software?
Fortiweb by Fortinet
Fortiweb by Fortinet
Fortiweb by Fortinet
Fortiweb by Fortinet
Fortiweb by Fortinet
⚠️ Risk & Real-World Impact
Worst Case
Complete system compromise with attacker gaining root/administrator access to the FortiWeb device, allowing them to pivot to internal networks, steal sensitive data, deploy ransomware, or establish persistent backdoors.
Likely Case
Remote code execution leading to device takeover, configuration modification, traffic interception, and lateral movement to connected systems.
If Mitigated
Limited impact if device is patched, network segmentation prevents lateral movement, and strict access controls are in place.
🎯 Exploit Status
The vulnerability requires no authentication and affects a core service, making exploitation straightforward for attackers with knowledge of the vulnerability.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: FortiWeb 6.0.8, 6.1.3, 6.2.7, 6.3.17, 6.4.1 and later
Vendor Advisory: https://fortiguard.com/psirt/FG-IR-21-186
Restart Required: Yes
Instructions:
1. Download the appropriate firmware update from the Fortinet support portal. 2. Backup current configuration. 3. Apply firmware update via web interface or CLI. 4. Reboot device. 5. Verify version and functionality.
🔧 Temporary Workarounds
Network Segmentation
allRestrict access to FortiWeb management interfaces and proxy services to trusted networks only.
Access Control Lists
allImplement strict firewall rules to limit which source IPs can communicate with FortiWeb proxy services.
🧯 If You Can't Patch
- Immediately isolate vulnerable FortiWeb devices from internet exposure
- Implement network-based intrusion prevention rules to block suspicious HTTP requests targeting FortiWeb
🔍 How to Verify
Check if Vulnerable:
Check FortiWeb firmware version via web interface (System > Status) or CLI command 'get system status'Check Version:
get system status | grep VersionVerify Fix Applied:
Confirm firmware version is 6.0.8+, 6.1.3+, 6.2.7+, 6.3.17+, or 6.4.1+📡 Detection & Monitoring
Log Indicators:
- Unusual proxy daemon crashes or restarts
- Suspicious HTTP requests with abnormal patterns or lengths
- Unexpected process execution events
Network Indicators:
- HTTP requests with abnormal payload sizes or patterns directed at FortiWeb proxy ports
- Traffic spikes to FortiWeb management interfaces from untrusted sources
SIEM Query:
source="fortiweb" AND (event_type="crash" OR http_request_size>threshold)